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re(3)                      Erlang Module Definition                      re(3)

NAME
       re - Perl like regular expressions for Erlang

DESCRIPTION
       This module contains regular expression matching functions for strings
       and binaries.

       The regular expression syntax and semantics resemble that of Perl.

       The library's matching algorithms are currently based on the PCRE
       library, but not all of the PCRE library is interfaced and some parts
       of the library go beyond what PCRE offers. The sections of the PCRE
       documentation which are relevant to this module are included here.

   Note:
       The Erlang literal syntax for strings uses the "\" (backslash)
       character as an escape code. You need to escape backslashes in literal
       strings, both in your code and in the shell, with an additional
       backslash, i.e.: "\\".

DATA TYPES

       mp() = {re_pattern, term(), term(), term(), term()}

              Opaque datatype containing a compiled regular expression. The
              mp() is guaranteed to be a tuple() having the atom 're_pattern'
              as its first element, to allow for matching in guards. The arity
              of the tuple() or the content of the other fields may change in
              future releases.

       nl_spec() = cr | crlf | lf | anycrlf | any

       compile_option() = unicode
                        | anchored
                        | caseless
                        | dollar_endonly
                        | dotall
                        | extended
                        | firstline
                        | multiline
                        | no_auto_capture
                        | dupnames
                        | ungreedy
                        | {newline, nl_spec()}
                        | bsr_anycrlf
                        | bsr_unicode
                        | no_start_optimize
                        | ucp
                        | never_utf

EXPORTS

       compile(Regexp) -> {ok, MP} | {error, ErrSpec}

              Types:

                 Regexp = iodata()
                 MP = mp()
                 ErrSpec =
                     {ErrString :: string(), Position :: integer() >= 0}

              The same as compile(Regexp,[])

       compile(Regexp, Options) -> {ok, MP} | {error, ErrSpec}

              Types:

                 Regexp = iodata() | unicode:charlist()
                 Options = [Option]
                 Option = compile_option()
                 MP = mp()
                 ErrSpec =
                     {ErrString :: string(), Position :: integer() >= 0}

              This function compiles a regular expression with the syntax
              described below into an internal format to be used later as a
              parameter to the run/2,3 functions.

              Compiling the regular expression before matching is useful if
              the same expression is to be used in matching against multiple
              subjects during the program's lifetime. Compiling once and
              executing many times is far more efficient than compiling each
              time one wants to match.

              When the unicode option is given, the regular expression should
              be given as a valid Unicode charlist(), otherwise as any valid
              iodata().

              The options have the following meanings:

                unicode:
                  The regular expression is given as a Unicode charlist() and
                  the resulting regular expression code is to be run against a
                  valid Unicode charlist() subject. Also consider the ucp
                  option when using Unicode characters.

                anchored:
                  The pattern is forced to be "anchored", that is, it is
                  constrained to match only at the first matching point in the
                  string that is being searched (the "subject string"). This
                  effect can also be achieved by appropriate constructs in the
                  pattern itself.

                caseless:
                  Letters in the pattern match both upper and lower case
                  letters. It is equivalent to Perl's /i option, and it can be
                  changed within a pattern by a (?i) option setting. Uppercase
                  and lowercase letters are defined as in the ISO-8859-1
                  character set.

                dollar_endonly:
                  A dollar metacharacter in the pattern matches only at the
                  end of the subject string. Without this option, a dollar
                  also matches immediately before a newline at the end of the
                  string (but not before any other newlines). The
                  dollar_endonly option is ignored if multiline is given.
                  There is no equivalent option in Perl, and no way to set it
                  within a pattern.

                dotall:
                  A dot in the pattern matches all characters, including those
                  that indicate newline. Without it, a dot does not match when
                  the current position is at a newline. This option is
                  equivalent to Perl's /s option, and it can be changed within
                  a pattern by a (?s) option setting. A negative class such as
                  [^a] always matches newline characters, independent of this
                  option's setting.

                extended:
                  Whitespace data characters in the pattern are ignored except
                  when escaped or inside a character class. Whitespace does
                  not include the VT character (ASCII 11). In addition,
                  characters between an unescaped # outside a character class
                  and the next newline, inclusive, are also ignored. This is
                  equivalent to Perl's /x option, and it can be changed within
                  a pattern by a (?x) option setting. This option makes it
                  possible to include comments inside complicated patterns.
                  Note, however, that this applies only to data characters.
                  Whitespace characters may never appear within special
                  character sequences in a pattern, for example within the
                  sequence (?( which introduces a conditional subpattern.

                firstline:
                  An unanchored pattern is required to match before or at the
                  first newline in the subject string, though the matched text
                  may continue over the newline.

                multiline:
                  By default, PCRE treats the subject string as consisting of
                  a single line of characters (even if it actually contains
                  newlines). The "start of line" metacharacter (^) matches
                  only at the start of the string, while the "end of line"
                  metacharacter ($) matches only at the end of the string, or
                  before a terminating newline (unless dollar_endonly is
                  given). This is the same as Perl.

                  When multiline is given, the "start of line" and "end of
                  line" constructs match immediately following or immediately
                  before internal newlines in the subject string,
                  respectively, as well as at the very start and end. This is
                  equivalent to Perl's /m option, and it can be changed within
                  a pattern by a (?m) option setting. If there are no newlines
                  in a subject string, or no occurrences of ^ or $ in a
                  pattern, setting multiline has no effect.

                no_auto_capture:
                  Disables the use of numbered capturing parentheses in the
                  pattern. Any opening parenthesis that is not followed by ?
                  behaves as if it were followed by ?: but named parentheses
                  can still be used for capturing (and they acquire numbers in
                  the usual way). There is no equivalent of this option in
                  Perl.

                dupnames:
                  Names used to identify capturing subpatterns need not be
                  unique. This can be helpful for certain types of pattern
                  when it is known that only one instance of the named
                  subpattern can ever be matched. There are more details of
                  named subpatterns below

                ungreedy:
                  This option inverts the "greediness" of the quantifiers so
                  that they are not greedy by default, but become greedy if
                  followed by "?". It is not compatible with Perl. It can also
                  be set by a (?U) option setting within the pattern.

                {newline, NLSpec}:
                  Override the default definition of a newline in the subject
                  string, which is LF (ASCII 10) in Erlang.

                  cr:
                    Newline is indicated by a single character CR (ASCII 13)

                  lf:
                    Newline is indicated by a single character LF (ASCII 10),
                    the default

                  crlf:
                    Newline is indicated by the two-character CRLF (ASCII 13
                    followed by ASCII 10) sequence.

                  anycrlf:
                    Any of the three preceding sequences should be recognized.

                  any:
                    Any of the newline sequences above, plus the Unicode
                    sequences VT (vertical tab, U+000B), FF (formfeed,
                    U+000C), NEL (next line, U+0085), LS (line separator,
                    U+2028), and PS (paragraph separator, U+2029).

                bsr_anycrlf:
                  Specifies specifically that \R is to match only the cr, lf
                  or crlf sequences, not the Unicode specific newline
                  characters.

                bsr_unicode:
                  Specifies specifically that \R is to match all the Unicode
                  newline characters (including crlf etc, the default).

                no_start_optimize:
                  This option disables optimization that may malfunction if
                  "Special start-of-pattern items" are present in the regular
                  expression. A typical example would be when matching
                  "DEFABC" against "(*COMMIT)ABC", where the start
                  optimization of PCRE would skip the subject up to the "A"
                  and would never realize that the (*COMMIT) instruction
                  should have made the matching fail. This option is only
                  relevant if you use "start-of-pattern items", as discussed
                  in the section "PCRE regular expression details" below.

                ucp:
                  Specifies that Unicode Character Properties should be used
                  when resolving \B, \b, \D, \d, \S, \s, \W and \w. Without
                  this flag, only ISO-Latin-1 properties are used. Using
                  Unicode properties hurts performance, but is semantically
                  correct when working with Unicode characters beyond the ISO-
                  Latin-1 range.

                never_utf:
                  Specifies that the (*UTF) and/or (*UTF8) "start-of-pattern
                  items" are forbidden. This flag can not be combined with
                  unicode. Useful if ISO-Latin-1 patterns from an external
                  source are to be compiled.

       inspect(MP, Item) -> {namelist, [binary()]}

              Types:

                 MP = mp()
                 Item = namelist

              This function takes a compiled regular expression and an item,
              returning the relevant data from the regular expression.
              Currently the only supported item is namelist, which returns the
              tuple {namelist, [ binary()]}, containing the names of all
              (unique) named subpatterns in the regular expression.

              Example:

              1> {ok,MP} = re:compile("(?<A>A)|(?<B>B)|(?<C>C)").
              {ok,{re_pattern,3,0,0,
                              <<69,82,67,80,119,0,0,0,0,0,0,0,1,0,0,0,255,255,255,255,
                                255,255,...>>}}
              2> re:inspect(MP,namelist).
              {namelist,[<<"A">>,<<"B">>,<<"C">>]}
              3> {ok,MPD} = re:compile("(?<C>A)|(?<B>B)|(?<C>C)",[dupnames]).
              {ok,{re_pattern,3,0,0,
                              <<69,82,67,80,119,0,0,0,0,0,8,0,1,0,0,0,255,255,255,255,
                                255,255,...>>}}
              4> re:inspect(MPD,namelist).
              {namelist,[<<"B">>,<<"C">>]}

              Note specifically in the second example that the duplicate name
              only occurs once in the returned list, and that the list is in
              alphabetical order regardless of where the names are positioned
              in the regular expression. The order of the names is the same as
              the order of captured subexpressions if {capture, all_names} is
              given as an option to re:run/3. You can therefore create a name-
              to-value mapping from the result of re:run/3 like this:

              1> {ok,MP} = re:compile("(?<A>A)|(?<B>B)|(?<C>C)").
              {ok,{re_pattern,3,0,0,
                              <<69,82,67,80,119,0,0,0,0,0,0,0,1,0,0,0,255,255,255,255,
                                255,255,...>>}}
              2> {namelist, N} = re:inspect(MP,namelist).
              {namelist,[<<"A">>,<<"B">>,<<"C">>]}
              3> {match,L} = re:run("AA",MP,[{capture,all_names,binary}]).
              {match,[<<"A">>,<<>>,<<>>]}
              4> NameMap = lists:zip(N,L).
              [{<<"A">>,<<"A">>},{<<"B">>,<<>>},{<<"C">>,<<>>}]

              More items are expected to be added in the future.

       run(Subject, RE) -> {match, Captured} | nomatch

              Types:

                 Subject = iodata() | unicode:charlist()
                 RE = mp() | iodata()
                 Captured = [CaptureData]
                 CaptureData = {integer(), integer()}

              The same as run(Subject,RE,[]).

       run(Subject, RE, Options) ->
              {match, Captured} | match | nomatch | {error, ErrType}

              Types:

                 Subject = iodata() | unicode:charlist()
                 RE = mp() | iodata() | unicode:charlist()
                 Options = [Option]
                 Option = anchored
                        | global
                        | notbol
                        | noteol
                        | notempty
                        | notempty_atstart
                        | report_errors
                        | {offset, integer() >= 0}
                        | {match_limit, integer() >= 0}
                        | {match_limit_recursion, integer() >= 0}
                        | {newline, NLSpec :: nl_spec()}
                        | bsr_anycrlf
                        | bsr_unicode
                        | {capture, ValueSpec}
                        | {capture, ValueSpec, Type}
                        | CompileOpt
                 Type = index | list | binary
                 ValueSpec = all
                           | all_but_first
                           | all_names
                           | first
                           | none
                           | ValueList
                 ValueList = [ValueID]
                 ValueID = integer() | string() | atom()
                 CompileOpt = compile_option()
                   See compile/2 above.
                 Captured = [CaptureData] | [[CaptureData]]
                 CaptureData = {integer(), integer()}
                             | ListConversionData
                             | binary()
                 ListConversionData = string()
                                    | {error, string(), binary()}
                                    | {incomplete, string(), binary()}
                 ErrType = match_limit
                         | match_limit_recursion
                         | {compile, CompileErr}
                 CompileErr =
                     {ErrString :: string(), Position :: integer() >= 0}

              Executes a regexp matching, returning match/{match, Captured} or
              nomatch. The regular expression can be given either as iodata()
              in which case it is automatically compiled (as by re:compile/2)
              and executed, or as a pre-compiled mp() in which case it is
              executed against the subject directly.

              When compilation is involved, the exception badarg is thrown if
              a compilation error occurs. Call re:compile/2 to get information
              about the location of the error in the regular expression.

              If the regular expression is previously compiled, the option
              list can only contain the options anchored, global, notbol,
              noteol, report_errors, notempty, notempty_atstart, {offset,
              integer() _= 0}, {match_limit, integer() _= 0},
              {match_limit_recursion, integer() _= 0}, {newline, NLSpec} and
              {capture, ValueSpec}/{capture, ValueSpec, Type}. Otherwise all
              options valid for the re:compile/2 function are allowed as well.
              Options allowed both for compilation and execution of a match,
              namely anchored and {newline, NLSpec}, will affect both the
              compilation and execution if present together with a non pre-
              compiled regular expression.

              If the regular expression was previously compiled with the
              option unicode, the Subject should be provided as a valid
              Unicode charlist(), otherwise any iodata() will do. If
              compilation is involved and the option unicode is given, both
              the Subject and the regular expression should be given as valid
              Unicode charlists().

              The {capture, ValueSpec}/{capture, ValueSpec, Type} defines what
              to return from the function upon successful matching. The
              capture tuple may contain both a value specification telling
              which of the captured substrings are to be returned, and a type
              specification, telling how captured substrings are to be
              returned (as index tuples, lists or binaries). The capture
              option makes the function quite flexible and powerful. The
              different options are described in detail below.

              If the capture options describe that no substring capturing at
              all is to be done ({capture, none}), the function will return
              the single atom match upon successful matching, otherwise the
              tuple {match, ValueList} is returned. Disabling capturing can be
              done either by specifying none or an empty list as ValueSpec.

              The report_errors option adds the possibility that an error
              tuple is returned. The tuple will either indicate a matching
              error (match_limit or match_limit_recursion) or a compilation
              error, where the error tuple has the format {error, {compile,
              CompileErr}}. Note that if the option report_errors is not
              given, the function never returns error tuples, but will report
              compilation errors as a badarg exception and failed matches due
              to exceeded match limits simply as nomatch.

              The options relevant for execution are:

                anchored:
                  Limits re:run/3 to matching at the first matching position.
                  If a pattern was compiled with anchored, or turned out to be
                  anchored by virtue of its contents, it cannot be made
                  unanchored at matching time, hence there is no unanchored
                  option.

                global:
                  Implements global (repetitive) search (the g flag in Perl).
                  Each match is returned as a separate list() containing the
                  specific match as well as any matching subexpressions (or as
                  specified by the capture option). The Captured part of the
                  return value will hence be a list() of list()s when this
                  option is given.

                  The interaction of the global option with a regular
                  expression which matches an empty string surprises some
                  users. When the global option is given, re:run/3 handles
                  empty matches in the same way as Perl: a zero-length match
                  at any point will be retried with the options [anchored,
                  notempty_atstart] as well. If that search gives a result of
                  length > 0, the result is included. For example:

                    re:run("cat","(|at)",[global]).

                  The following matching will be performed:

                  At offset 0:
                    The regexp (|at) will first match at the initial position
                    of the string cat, giving the result set [{0,0},{0,0}]
                    (the second {0,0} is due to the subexpression marked by
                    the parentheses). As the length of the match is 0, we
                    don't advance to the next position yet.

                  At offset 0 with [anchored, notempty_atstart]:
                     The search is retried with the options [anchored,
                    notempty_atstart] at the same position, which does not
                    give any interesting result of longer length, so the
                    search position is now advanced to the next character (a).

                  At offset 1:
                    This time, the search results in [{1,0},{1,0}], so this
                    search will also be repeated with the extra options.

                  At offset 1 with [anchored, notempty_atstart]:
                    Now the ab alternative is found and the result will be
                    [{1,2},{1,2}]. The result is added to the list of results
                    and the position in the search string is advanced two
                    steps.

                  At offset 3:
                    The search now once again matches the empty string, giving
                    [{3,0},{3,0}].

                  At offset 1 with [anchored, notempty_atstart]:
                    This will give no result of length > 0 and we are at the
                    last position, so the global search is complete.

                  The result of the call is:

                     {match,[[{0,0},{0,0}],[{1,0},{1,0}],[{1,2},{1,2}],[{3,0},{3,0}]]}

                notempty:
                  An empty string is not considered to be a valid match if
                  this option is given. If there are alternatives in the
                  pattern, they are tried. If all the alternatives match the
                  empty string, the entire match fails. For example, if the
                  pattern

                    a?b?

                  is applied to a string not beginning with "a" or "b", it
                  would normally match the empty string at the start of the
                  subject. With the notempty option, this match is not valid,
                  so re:run/3 searches further into the string for occurrences
                  of "a" or "b".

                notempty_atstart:
                  This is like notempty, except that an empty string match
                  that is not at the start of the subject is permitted. If the
                  pattern is anchored, such a match can occur only if the
                  pattern contains \K.

                  Perl has no direct equivalent of notempty or
                  notempty_atstart, but it does make a special case of a
                  pattern match of the empty string within its split()
                  function, and when using the /g modifier. It is possible to
                  emulate Perl's behavior after matching a null string by
                  first trying the match again at the same offset with
                  notempty_atstart and anchored, and then, if that fails, by
                  advancing the starting offset (see below) and trying an
                  ordinary match again.

                notbol:
                  This option specifies that the first character of the
                  subject string is not the beginning of a line, so the
                  circumflex metacharacter should not match before it. Setting
                  this without multiline (at compile time) causes circumflex
                  never to match. This option only affects the behavior of the
                  circumflex metacharacter. It does not affect \A.

                noteol:
                  This option specifies that the end of the subject string is
                  not the end of a line, so the dollar metacharacter should
                  not match it nor (except in multiline mode) a newline
                  immediately before it. Setting this without multiline (at
                  compile time) causes dollar never to match. This option
                  affects only the behavior of the dollar metacharacter. It
                  does not affect \Z or \z.

                report_errors:
                  This option gives better control of the error handling in
                  re:run/3. When it is given, compilation errors (if the
                  regular expression isn't already compiled) as well as run-
                  time errors are explicitly returned as an error tuple.

                  The possible run-time errors are:

                  match_limit:
                    The PCRE library sets a limit on how many times the
                    internal match function can be called. The default value
                    for this is 10000000 in the library compiled for Erlang.
                    If {error, match_limit} is returned, it means that the
                    execution of the regular expression has reached this
                    limit. Normally this is to be regarded as a nomatch, which
                    is the default return value when this happens, but by
                    specifying report_errors, you will get informed when the
                    match fails due to to many internal calls.

                  match_limit_recursion:
                    This error is very similar to match_limit, but occurs when
                    the internal match function of PCRE is "recursively"
                    called more times than the "match_limit_recursion" limit,
                    which is by default 10000000 as well. Note that as long as
                    the match_limit and match_limit_default values are kept at
                    the default values, the match_limit_recursion error can
                    not occur, as the match_limit error will occur before that
                    (each recursive call is also a call, but not vice versa).
                    Both limits can however be changed, either by setting
                    limits directly in the regular expression string (see
                    reference section below) or by giving options to re:run/3

                  It is important to understand that what is referred to as
                  "recursion" when limiting matches is not actually recursion
                  on the C stack of the Erlang machine, neither is it
                  recursion on the Erlang process stack. The version of PCRE
                  compiled into the Erlang VM uses machine "heap" memory to
                  store values that needs to be kept over recursion in regular
                  expression matches.

                {match_limit, integer() _= 0}:
                  This option limits the execution time of a match in an
                  implementation-specific way. It is described in the
                  following way by the PCRE documentation:

                The match_limit field provides a means of preventing PCRE from using
                up a vast amount of resources when running patterns that are not going
                to match, but which have a very large number of possibilities in their
                search trees. The classic example is a pattern that uses nested
                unlimited repeats.

                Internally, pcre_exec() uses a function called match(), which it calls
                repeatedly (sometimes recursively). The limit set by match_limit is
                imposed on the number of times this function is called during a match,
                which has the effect of limiting the amount of backtracking that can
                take place. For patterns that are not anchored, the count restarts
                from zero for each position in the subject string.

                  This means that runaway regular expression matches can fail
                  faster if the limit is lowered using this option. The
                  default value compiled into the Erlang virtual machine is
                  10000000

            Note:
                This option does in no way affect the execution of the Erlang
                virtual machine in terms of "long running BIF's". re:run
                always give control back to the scheduler of Erlang processes
                at intervals that ensures the real time properties of the
                Erlang system.

                {match_limit_recursion, integer() _= 0}:
                  This option limits the execution time and memory consumption
                  of a match in an implementation-specific way, very similar
                  to match_limit. It is described in the following way by the
                  PCRE documentation:

                The match_limit_recursion field is similar to match_limit, but instead
                of limiting the total number of times that match() is called, it
                limits the depth of recursion. The recursion depth is a smaller number
                than the total number of calls, because not all calls to match() are
                recursive. This limit is of use only if it is set smaller than
                match_limit.

                Limiting the recursion depth limits the amount of machine stack that
                can be used, or, when PCRE has been compiled to use memory on the heap
                instead of the stack, the amount of heap memory that can be
                used.

                  The Erlang virtual machine uses a PCRE library where heap
                  memory is used when regular expression match recursion
                  happens, why this limits the usage of machine heap, not C
                  stack.

                  Specifying a lower value may result in matches with deep
                  recursion failing, when they should actually have matched:

                1> re:run("aaaaaaaaaaaaaz","(a+)*z").
                {match,[{0,14},{0,13}]}
                2> re:run("aaaaaaaaaaaaaz","(a+)*z",[{match_limit_recursion,5}]).
                nomatch
                3> re:run("aaaaaaaaaaaaaz","(a+)*z",[{match_limit_recursion,5},report_errors]).
                {error,match_limit_recursion}

                  This option, as well as the match_limit option should only
                  be used in very rare cases. Understanding of the PCRE
                  library internals is recommended before tampering with these
                  limits.

                {offset, integer() _= 0}:
                  Start matching at the offset (position) given in the subject
                  string. The offset is zero-based, so that the default is
                  {offset,0} (all of the subject string).

                {newline, NLSpec}:
                  Override the default definition of a newline in the subject
                  string, which is LF (ASCII 10) in Erlang.

                  cr:
                    Newline is indicated by a single character CR (ASCII 13)

                  lf:
                    Newline is indicated by a single character LF (ASCII 10),
                    the default

                  crlf:
                    Newline is indicated by the two-character CRLF (ASCII 13
                    followed by ASCII 10) sequence.

                  anycrlf:
                    Any of the three preceding sequences should be recognized.

                  any:
                    Any of the newline sequences above, plus the Unicode
                    sequences VT (vertical tab, U+000B), FF (formfeed,
                    U+000C), NEL (next line, U+0085), LS (line separator,
                    U+2028), and PS (paragraph separator, U+2029).

                bsr_anycrlf:
                  Specifies specifically that \R is to match only the cr, lf
                  or crlf sequences, not the Unicode specific newline
                  characters. (overrides compilation option)

                bsr_unicode:
                  Specifies specifically that \R is to match all the Unicode
                  newline characters (including crlf etc, the
                  default).(overrides compilation option)

                {capture, ValueSpec}/{capture, ValueSpec, Type}:
                  Specifies which captured substrings are returned and in what
                  format. By default, re:run/3 captures all of the matching
                  part of the substring as well as all capturing subpatterns
                  (all of the pattern is automatically captured). The default
                  return type is (zero-based) indexes of the captured parts of
                  the string, given as {Offset,Length} pairs (the index Type
                  of capturing).

                  As an example of the default behavior, the following call:

                    re:run("ABCabcdABC","abcd",[]).

                  returns, as first and only captured string the matching part
                  of the subject ("abcd" in the middle) as a index pair {3,4},
                  where character positions are zero based, just as in
                  offsets. The return value of the call above would then be:

                    {match,[{3,4}]}

                  Another (and quite common) case is where the regular
                  expression matches all of the subject, as in:

                    re:run("ABCabcdABC",".*abcd.*",[]).

                  where the return value correspondingly will point out all of
                  the string, beginning at index 0 and being 10 characters
                  long:

                    {match,[{0,10}]}

                  If the regular expression contains capturing subpatterns,
                  like in the following case:

                    re:run("ABCabcdABC",".*(abcd).*",[]).

                  all of the matched subject is captured, as well as the
                  captured substrings:

                    {match,[{0,10},{3,4}]}

                  the complete matching pattern always giving the first return
                  value in the list and the rest of the subpatterns being
                  added in the order they occurred in the regular expression.

                  The capture tuple is built up as follows:

                  ValueSpec:
                    Specifies which captured (sub)patterns are to be returned.
                    The ValueSpec can either be an atom describing a
                    predefined set of return values, or a list containing
                    either the indexes or the names of specific subpatterns to
                    return.

                    The predefined sets of subpatterns are:

                    all:
                      All captured subpatterns including the complete matching
                      string. This is the default.

                    all_names:
                      All named subpatterns in the regular expression, as if a
                      list() of all the names in alphabetical order was given.
                      The list of all names can also be retrieved with the
                      inspect/2 function.

                    first:
                      Only the first captured subpattern, which is always the
                      complete matching part of the subject. All explicitly
                      captured subpatterns are discarded.

                    all_but_first:
                      All but the first matching subpattern, i.e. all
                      explicitly captured subpatterns, but not the complete
                      matching part of the subject string. This is useful if
                      the regular expression as a whole matches a large part
                      of the subject, but the part you're interested in is in
                      an explicitly captured subpattern. If the return type is
                      list or binary, not returning subpatterns you're not
                      interested in is a good way to optimize.

                    none:
                      Do not return matching subpatterns at all, yielding the
                      single atom match as the return value of the function
                      when matching successfully instead of the {match,
                      list()} return. Specifying an empty list gives the same
                      behavior.

                    The value list is a list of indexes for the subpatterns to
                    return, where index 0 is for all of the pattern, and 1 is
                    for the first explicit capturing subpattern in the regular
                    expression, and so forth. When using named captured
                    subpatterns (see below) in the regular expression, one can
                    use atom()s or string()s to specify the subpatterns to be
                    returned. For example, consider the regular expression:

                      ".*(abcd).*"

                    matched against the string "ABCabcdABC", capturing only
                    the "abcd" part (the first explicit subpattern):

                      re:run("ABCabcdABC",".*(abcd).*",[{capture,[1]}]).

                    The call will yield the following result:

                      {match,[{3,4}]}

                    as the first explicitly captured subpattern is "(abcd)",
                    matching "abcd" in the subject, at (zero-based) position
                    3, of length 4.

                    Now consider the same regular expression, but with the
                    subpattern explicitly named 'FOO':

                      ".*(?<FOO>abcd).*"

                    With this expression, we could still give the index of the
                    subpattern with the following call:

                      re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,[1]}]).

                    giving the same result as before. But, since the
                    subpattern is named, we can also specify its name in the
                    value list:

                      re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,['FOO']}]).

                    which would yield the same result as the earlier examples,
                    namely:

                      {match,[{3,4}]}

                    The values list might specify indexes or names not present
                    in the regular expression, in which case the return values
                    vary depending on the type. If the type is index, the
                    tuple {-1,0} is returned for values having no
                    corresponding subpattern in the regexp, but for the other
                    types (binary and list), the values are the empty binary
                    or list respectively.

                  Type:
                    Optionally specifies how captured substrings are to be
                    returned. If omitted, the default of index is used. The
                    Type can be one of the following:

                    index:
                      Return captured substrings as pairs of byte indexes into
                      the subject string and length of the matching string in
                      the subject (as if the subject string was flattened with
                      iolist_to_binary/1 or unicode:characters_to_binary/2
                      prior to matching). Note that the unicode option results
                      in byte-oriented indexes in a (possibly virtual) UTF-8
                      encoded binary. A byte index tuple {0,2} might therefore
                      represent one or two characters when unicode is in
                      effect. This might seem counter-intuitive, but has been
                      deemed the most effective and useful way to way to do
                      it. To return lists instead might result in simpler code
                      if that is desired. This return type is the default.

                    list:
                      Return matching substrings as lists of characters
                      (Erlang string()s). It the unicode option is used in
                      combination with the \C sequence in the regular
                      expression, a captured subpattern can contain bytes that
                      are not valid UTF-8 (\C matches bytes regardless of
                      character encoding). In that case the list capturing may
                      result in the same types of tuples that
                      unicode:characters_to_list/2 can return, namely three-
                      tuples with the tag incomplete or error, the
                      successfully converted characters and the invalid UTF-8
                      tail of the conversion as a binary. The best strategy is
                      to avoid using the \C sequence when capturing lists.

                    binary:
                      Return matching substrings as binaries. If the unicode
                      option is used, these binaries are in UTF-8. If the \C
                      sequence is used together with unicode the binaries may
                      be invalid UTF-8.

                  In general, subpatterns that were not assigned a value in
                  the match are returned as the tuple {-1,0} when type is
                  index. Unassigned subpatterns are returned as the empty
                  binary or list, respectively, for other return types.
                  Consider the regular expression:

                    ".*((?<FOO>abdd)|a(..d)).*"

                  There are three explicitly capturing subpatterns, where the
                  opening parenthesis position determines the order in the
                  result, hence ((?_FOO_abdd)|a(..d)) is subpattern index 1,
                  (?_FOO_abdd) is subpattern index 2 and (..d) is subpattern
                  index 3. When matched against the following string:

                    "ABCabcdABC"

                  the subpattern at index 2 won't match, as "abdd" is not
                  present in the string, but the complete pattern matches (due
                  to the alternative a(..d). The subpattern at index 2 is
                  therefore unassigned and the default return value will be:

                    {match,[{0,10},{3,4},{-1,0},{4,3}]}

                  Setting the capture Type to binary would give the following:

                    {match,[<<"ABCabcdABC">>,<<"abcd">>,<<>>,<<"bcd">>]}

                  where the empty binary (____) represents the unassigned
                  subpattern. In the binary case, some information about the
                  matching is therefore lost, the ____ might just as well be
                  an empty string captured.

                  If differentiation between empty matches and non existing
                  subpatterns is necessary, use the type index and do the
                  conversion to the final type in Erlang code.

                  When the option global is given, the capture specification
                  affects each match separately, so that:

                    re:run("cacb","c(a|b)",[global,{capture,[1],list}]).

                  gives the result:

                    {match,[["a"],["b"]]}

              The options solely affecting the compilation step are described
              in the re:compile/2 function.

       replace(Subject, RE, Replacement) -> iodata() | unicode:charlist()

              Types:

                 Subject = iodata() | unicode:charlist()
                 RE = mp() | iodata()
                 Replacement = iodata() | unicode:charlist()

              The same as replace(Subject,RE,Replacement,[]).

       replace(Subject, RE, Replacement, Options) ->
                  iodata() | unicode:charlist()

              Types:

                 Subject = iodata() | unicode:charlist()
                 RE = mp() | iodata() | unicode:charlist()
                 Replacement = iodata() | unicode:charlist()
                 Options = [Option]
                 Option = anchored
                        | global
                        | notbol
                        | noteol
                        | notempty
                        | notempty_atstart
                        | {offset, integer() >= 0}
                        | {newline, NLSpec}
                        | bsr_anycrlf
                        | {match_limit, integer() >= 0}
                        | {match_limit_recursion, integer() >= 0}
                        | bsr_unicode
                        | {return, ReturnType}
                        | CompileOpt
                 ReturnType = iodata | list | binary
                 CompileOpt = compile_option()
                 NLSpec = cr | crlf | lf | anycrlf | any

              Replaces the matched part of the Subject string with the
              contents of Replacement.

              The permissible options are the same as for re:run/3, except
              that the capture option is not allowed. Instead a {return,
              ReturnType} is present. The default return type is iodata,
              constructed in a way to minimize copying. The iodata result can
              be used directly in many I/O-operations. If a flat list() is
              desired, specify {return, list} and if a binary is preferred,
              specify {return, binary}.

              As in the re:run/3 function, an mp() compiled with the unicode
              option requires the Subject to be a Unicode charlist(). If
              compilation is done implicitly and the unicode compilation
              option is given to this function, both the regular expression
              and the Subject should be given as valid Unicode charlist()s.

              The replacement string can contain the special character _,
              which inserts the whole matching expression in the result, and
              the special sequence \N (where N is an integer > 0), \gN or
              \g{N} resulting in the subexpression number N will be inserted
              in the result. If no subexpression with that number is generated
              by the regular expression, nothing is inserted.

              To insert an _ or \ in the result, precede it with a \. Note
              that Erlang already gives a special meaning to \ in literal
              strings, so a single \ has to be written as "\\" and therefore a
              double \ as "\\\\". Example:

                  re:replace("abcd","c","[&]",[{return,list}]).

              gives

                  "ab[c]d"

              while

                  re:replace("abcd","c","[\\&]",[{return,list}]).

              gives

                  "ab[&]d"

              As with re:run/3, compilation errors raise the badarg exception,
              re:compile/2 can be used to get more information about the
              error.

       split(Subject, RE) -> SplitList

              Types:

                 Subject = iodata() | unicode:charlist()
                 RE = mp() | iodata()
                 SplitList = [iodata() | unicode:charlist()]

              The same as split(Subject,RE,[]).

       split(Subject, RE, Options) -> SplitList

              Types:

                 Subject = iodata() | unicode:charlist()
                 RE = mp() | iodata() | unicode:charlist()
                 Options = [Option]
                 Option = anchored
                        | notbol
                        | noteol
                        | notempty
                        | notempty_atstart
                        | {offset, integer() >= 0}
                        | {newline, nl_spec()}
                        | {match_limit, integer() >= 0}
                        | {match_limit_recursion, integer() >= 0}
                        | bsr_anycrlf
                        | bsr_unicode
                        | {return, ReturnType}
                        | {parts, NumParts}
                        | group
                        | trim
                        | CompileOpt
                 NumParts = integer() >= 0 | infinity
                 ReturnType = iodata | list | binary
                 CompileOpt = compile_option()
                   See compile/2 above.
                 SplitList = [RetData] | [GroupedRetData]
                 GroupedRetData = [RetData]
                 RetData = iodata() | unicode:charlist() | binary() | list()

              This function splits the input into parts by finding tokens
              according to the regular expression supplied.

              The splitting is done basically by running a global regexp match
              and dividing the initial string wherever a match occurs. The
              matching part of the string is removed from the output.

              As in the re:run/3 function, an mp() compiled with the unicode
              option requires the Subject to be a Unicode charlist(). If
              compilation is done implicitly and the unicode compilation
              option is given to this function, both the regular expression
              and the Subject should be given as valid Unicode charlist()s.

              The result is given as a list of "strings", the preferred
              datatype given in the return option (default iodata).

              If subexpressions are given in the regular expression, the
              matching subexpressions are returned in the resulting list as
              well. An example:

                  re:split("Erlang","[ln]",[{return,list}]).

              will yield the result:

                  ["Er","a","g"]

              while

                  re:split("Erlang","([ln])",[{return,list}]).

              will yield

                  ["Er","l","a","n","g"]

              The text matching the subexpression (marked by the parentheses
              in the regexp) is inserted in the result list where it was
              found. In effect this means that concatenating the result of a
              split where the whole regexp is a single subexpression (as in
              the example above) will always result in the original string.

              As there is no matching subexpression for the last part in the
              example (the "g"), there is nothing inserted after that. To make
              the group of strings and the parts matching the subexpressions
              more obvious, one might use the group option, which groups
              together the part of the subject string with the parts matching
              the subexpressions when the string was split:

                  re:split("Erlang","([ln])",[{return,list},group]).

              gives:

                  [["Er","l"],["a","n"],["g"]]

              Here the regular expression matched first the "l", causing "Er"
              to be the first part in the result. When the regular expression
              matched, the (only) subexpression was bound to the "l", so the
              "l" is inserted in the group together with "Er". The next match
              is of the "n", making "a" the next part to be returned. Since
              the subexpression is bound to the substring "n" in this case,
              the "n" is inserted into this group. The last group consists of
              the rest of the string, as no more matches are found.

              By default, all parts of the string, including the empty
              strings, are returned from the function. For example:

                  re:split("Erlang","[lg]",[{return,list}]).

              will return:

                  ["Er","an",[]]

              since the matching of the "g" in the end of the string leaves an
              empty rest which is also returned. This behaviour differs from
              the default behaviour of the split function in Perl, where empty
              strings at the end are by default removed. To get the "trimming"
              default behavior of Perl, specify trim as an option:

                  re:split("Erlang","[lg]",[{return,list},trim]).

              The result will be:

                  ["Er","an"]

              The "trim" option in effect says; "give me as many parts as
              possible except the empty ones", which might be useful in some
              circumstances. You can also specify how many parts you want, by
              specifying {parts,N}:

                  re:split("Erlang","[lg]",[{return,list},{parts,2}]).

              This will give:

                  ["Er","ang"]

              Note that the last part is "ang", not "an", as we only specified
              splitting into two parts, and the splitting stops when enough
              parts are given, which is why the result differs from that of
              trim.

              More than three parts are not possible with this indata, so

                  re:split("Erlang","[lg]",[{return,list},{parts,4}]).

              will give the same result as the default, which is to be viewed
              as "an infinite number of parts".

              Specifying 0 as the number of parts gives the same effect as the
              option trim. If subexpressions are captured, empty subexpression
              matches at the end are also stripped from the result if trim or
              {parts,0} is specified.

              If you are familiar with Perl, the trim behaviour corresponds
              exactly to the Perl default, the {parts,N} where N is a positive
              integer corresponds exactly to the Perl behaviour with a
              positive numerical third parameter and the default behaviour of
              re:split/3 corresponds to that when the Perl routine is given a
              negative integer as the third parameter.

              Summary of options not previously described for the re:run/3
              function:

                {return,ReturnType}:
                  Specifies how the parts of the original string are presented
                  in the result list. The possible types are:

                  iodata:
                    The variant of iodata() that gives the least copying of
                    data with the current implementation (often a binary, but
                    don't depend on it).

                  binary:
                    All parts returned as binaries.

                  list:
                    All parts returned as lists of characters ("strings").

                group:
                  Groups together the part of the string with the parts of the
                  string matching the subexpressions of the regexp.

                  The return value from the function will in this case be a
                  list() of list()s. Each sublist begins with the string
                  picked out of the subject string, followed by the parts
                  matching each of the subexpressions in order of occurrence
                  in the regular expression.

                {parts,N}:
                  Specifies the number of parts the subject string is to be
                  split into.

                  The number of parts should be a positive integer for a
                  specific maximum on the number of parts and infinity for the
                  maximum number of parts possible (the default). Specifying
                  {parts,0} gives as many parts as possible disregarding empty
                  parts at the end, the same as specifying trim

                trim:
                  Specifies that empty parts at the end of the result list are
                  to be disregarded. The same as specifying {parts,0}. This
                  corresponds to the default behaviour of the split built in
                  function in Perl.

PERL LIKE REGULAR EXPRESSIONS SYNTAX
       The following sections contain reference material for the regular
       expressions used by this module. The regular expression reference is
       based on the PCRE documentation, with changes in cases where the re
       module behaves differently to the PCRE library.

PCRE REGULAR EXPRESSION DETAILS
       The syntax and semantics of the regular expressions that are supported
       by PCRE are described in detail below. Perl's regular expressions are
       described in its own documentation, and regular expressions in general
       are covered in a number of books, some of which have copious examples.
       Jeffrey Friedl's "Mastering Regular Expressions", published by
       O'Reilly, covers regular expressions in great detail. This description
       of PCRE's regular expressions is intended as reference material.

       The reference material is divided into the following sections:

         * Special start-of-pattern items

         * Characters and metacharacters

         * Backslash

         * Circumflex and dollar

         * Full stop (period, dot) and \N

         * Matching a single data unit

         * Square brackets and character classes

         * POSIX character classes

         * Vertical bar

         * Internal option setting

         * Subpatterns

         * Duplicate subpattern numbers

         * Named subpatterns

         * Repetition

         * Atomic grouping and possessive quantifiers

         * Back references

         * Assertions

         * Conditional subpatterns

         * Comments

         * Recursive patterns

         * Subpatterns as subroutines

         * Oniguruma subroutine syntax

         * Backtracking control

SPECIAL START-OF-PATTERN ITEMS
       A number of options that can be passed to re:compile/2 can also be set
       by special items at the start of a pattern. These are not Perl-
       compatible, but are provided to make these options accessible to
       pattern writers who are not able to change the program that processes
       the pattern. Any number of these items may appear, but they must all be
       together right at the start of the pattern string, and the letters must
       be in upper case.

       UTF support

       Unicode support is basically UTF-8 based. To use Unicode characters,
       you either call re:compile/2/re:run/3 with the unicode option, or the
       pattern must start with one of these special sequences:

       (*UTF8)

       (*UTF)

       Both options give the same effect, the input string is interpreted as
       UTF-8. Note that with these instructions, the automatic conversion of
       lists to UTF-8 is not performed by the re functions, why using these
       options is not recommended. Add the unicode option when running
       re:compile/2 instead.

       Some applications that allow their users to supply patterns may wish to
       restrict them to non-UTF data for security reasons. If the never_utf
       option is set at compile time, (*UTF) etc. are not allowed, and their
       appearance causes an error.

       Unicode property support

       Another special sequence that may appear at the start of a pattern is

       (*UCP)

       This has the same effect as setting the ucp option: it causes sequences
       such as \d and \w to use Unicode properties to determine character
       types, instead of recognizing only characters with codes less than 256
       via a lookup table.

       Disabling start-up optimizations

       If a pattern starts with (*NO_START_OPT), it has the same effect as
       setting the no_Start_optimize option at compile time.

       Newline conventions

       PCRE supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a single LF
       (linefeed) character, the two-character sequence CRLF , any of the
       three preceding, or any Unicode newline sequence.

       It is also possible to specify a newline convention by starting a
       pattern string with one of the following five sequences:

         (*CR):
           carriage return

         (*LF):
           linefeed

         (*CRLF):
           carriage return, followed by linefeed

         (*ANYCRLF):
           any of the three above

         (*ANY):
           all Unicode newline sequences

       These override the default and the options given to re:compile/2. For
       example, the pattern:

       (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no longer a newline. If more than one of them is present, the last one
       is used.

       The newline convention affects where the circumflex and dollar
       assertions are true. It also affects the interpretation of the dot
       metacharacter when dotall is not set, and the behaviour of \N. However,
       it does not affect what the \R escape sequence matches. By default,
       this is any Unicode newline sequence, for Perl compatibility. However,
       this can be changed; see the description of \R in the section entitled
       "Newline sequences" below. A change of \R setting can be combined with
       a change of newline convention.

       Setting match and recursion limits

       The caller of re:run/3 can set a limit on the number of times the
       internal match() function is called and on the maximum depth of
       recursive calls. These facilities are provided to catch runaway matches
       that are provoked by patterns with huge matching trees (a typical
       example is a pattern with nested unlimited repeats) and to avoid
       running out of system stack by too much recursion. When one of these
       limits is reached, pcre_exec() gives an error return. The limits can
       also be set by items at the start of the pattern of the form

       (*LIMIT_MATCH=d)

       (*LIMIT_RECURSION=d)

       where d is any number of decimal digits. However, the value of the
       setting must be less than the value set by the caller of re:run/3 for
       it to have any effect. In other words, the pattern writer can lower the
       limit set by the programmer, but not raise it. If there is more than
       one setting of one of these limits, the lower value is used.

       The current default value for both the limits are 10000000 in the
       Erlang VM. Note that the recursion limit does not actually affect the
       stack depth of the VM, as PCRE for Erlang is compiled in such a way
       that the match function never does recursion on the "C-stack".

CHARACTERS AND METACHARACTERS
       A regular expression is a pattern that is matched against a subject
       string from left to right. Most characters stand for themselves in a
       pattern, and match the corresponding characters in the subject. As a
       trivial example, the pattern

       The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless matching is specified (the caseless option), letters are
       matched independently of case.

       The power of regular expressions comes from the ability to include
       alternatives and repetitions in the pattern. These are encoded in the
       pattern by the use of metacharacters, which do not stand for themselves
       but instead are interpreted in some special way.

       There are two different sets of metacharacters: those that are
       recognized anywhere in the pattern except within square brackets, and
       those that are recognized within square brackets. Outside square
       brackets, the metacharacters are as follows:

         \:
           general escape character with several uses

         ^:
           assert start of string (or line, in multiline mode)

         $:
           assert end of string (or line, in multiline mode)

         .:
           match any character except newline (by default)

         [:
           start character class definition

         |:
           start of alternative branch

         (:
           start subpattern

         ):
           end subpattern

         ?:
           extends the meaning of (, also 0 or 1 quantifier, also quantifier
           minimizer

         *:
           0 or more quantifier

         +:
           1 or more quantifier, also "possessive quantifier"

         {:
           start min/max quantifier

       Part of a pattern that is in square brackets is called a "character
       class". In a character class the only metacharacters are:

         \:
           general escape character

         ^:
           negate the class, but only if the first character

         -:
           indicates character range

         [:
           POSIX character class (only if followed by POSIX syntax)

         ]:
           terminates the character class

       The following sections describe the use of each of the metacharacters.

BACKSLASH
       The backslash character has several uses. Firstly, if it is followed by
       a character that is not a number or a letter, it takes away any special
       meaning that character may have. This use of backslash as an escape
       character applies both inside and outside character classes.

       For example, if you want to match a * character, you write \* in the
       pattern. This escaping action applies whether or not the following
       character would otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric with backslash to specify
       that it stands for itself. In particular, if you want to match a
       backslash, you write \\.

       In unicode mode, only ASCII numbers and letters have any special
       meaning after a backslash. All other characters (in particular, those
       whose codepoints are greater than 127) are treated as literals.

       If a pattern is compiled with the extended option, white space in the
       pattern (other than in a character class) and characters between a #
       outside a character class and the next newline are ignored. An escaping
       backslash can be used to include a white space or # character as part
       of the pattern.

       If you want to remove the special meaning from a sequence of
       characters, you can do so by putting them between \Q and \E. This is
       different from Perl in that $ and @ are handled as literals in \Q...\E
       sequences in PCRE, whereas in Perl, $ and @ cause variable
       interpolation. Note the following examples:

         Pattern           PCRE matches   Perl matches

         \Qabc$xyz\E       abc$xyz        abc followed by the contents of $xyz
         \Qabc\$xyz\E      abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E  abc$xyz        abc$xyz

       The \Q...\E sequence is recognized both inside and outside character
       classes. An isolated \E that is not preceded by \Q is ignored. If \Q is
       not followed by \E later in the pattern, the literal interpretation
       continues to the end of the pattern (that is, \E is assumed at the
       end). If the isolated \Q is inside a character class, this causes an
       error, because the character class is not terminated.

       Non-printing characters

       A second use of backslash provides a way of encoding non-printing
       characters in patterns in a visible manner. There is no restriction on
       the appearance of non-printing characters, apart from the binary zero
       that terminates a pattern, but when a pattern is being prepared by text
       editing, it is often easier to use one of the following escape
       sequences than the binary character it represents:

         \a:
           alarm, that is, the BEL character (hex 07)

         \cx:
           "control-x", where x is any ASCII character

         \e :
           escape (hex 1B)

         \f:
           form feed (hex 0C)

         \n:
           linefeed (hex 0A)

         \r:
           carriage return (hex 0D)

         \t :
           tab (hex 09)

         \ddd:
           character with octal code ddd, or back reference

         \xhh :
           character with hex code hh

         \x{hhh..}:
           character with hex code hhh..

       The precise effect of \cx on ASCII characters is as follows: if x is a
       lower case letter, it is converted to upper case. Then bit 6 of the
       character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A
       (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes
       hex 7B (; is 3B). If the data item (byte or 16-bit value) following \c
       has a value greater than 127, a compile-time error occurs. This locks
       out non-ASCII characters in all modes.

       The \c facility was designed for use with ASCII characters, but with
       the extension to Unicode it is even less useful than it once was.

       By default, after \x, from zero to two hexadecimal digits are read
       (letters can be in upper or lower case). Any number of hexadecimal
       digits may appear between \x{ and }, but the character code is
       constrained as follows:

         8-bit non-Unicode mode:
           less than 0x100

         8-bit UTF-8 mode:
           less than 0x10ffff and a valid codepoint

       Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-
       called "surrogate" codepoints), and 0xffef.

       If characters other than hexadecimal digits appear between \x{ and },
       or if there is no terminating }, this form of escape is not recognized.
       Instead, the initial \x will be interpreted as a basic hexadecimal
       escape, with no following digits, giving a character whose value is
       zero.

       Characters whose value is less than 256 can be defined by either of the
       two syntaxes for \x. There is no difference in the way they are
       handled. For example, \xdc is exactly the same as \x{dc}.

       After \0 up to two further octal digits are read. If there are fewer
       than two digits, just those that are present are used. Thus the
       sequence \0\x\07 specifies two binary zeros followed by a BEL character
       (code value 7). Make sure you supply two digits after the initial zero
       if the pattern character that follows is itself an octal digit.

       The handling of a backslash followed by a digit other than 0 is
       complicated. Outside a character class, PCRE reads it and any following
       digits as a decimal number. If the number is less than 10, or if there
       have been at least that many previous capturing left parentheses in the
       expression, the entire sequence is taken as a back reference. A
       description of how this works is given later, following the discussion
       of parenthesized subpatterns.

       Inside a character class, or if the decimal number is greater than 9
       and there have not been that many capturing subpatterns, PCRE re-reads
       up to three octal digits following the backslash, and uses them to
       generate a data character. Any subsequent digits stand for themselves.
       The value of the character is constrained in the same way as characters
       specified in hexadecimal. For example:

         \040:
           is another way of writing a ASCII space

         \40:
           is the same, provided there are fewer than 40 previous capturing
           subpatterns

         \7:
           is always a back reference

         \11:
            might be a back reference, or another way of writing a tab

         \011:
           is always a tab

         \0113:
           is a tab followed by the character "3"

         \113:
           might be a back reference, otherwise the character with octal code
           113

         \377:
           might be a back reference, otherwise the value 255 (decimal)

         \81:
           is either a back reference, or a binary zero followed by the two
           characters "8" and "1"

       Note that octal values of 100 or greater must not be introduced by a
       leading zero, because no more than three octal digits are ever read.

       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside a character
       class, \b is interpreted as the backspace character (hex 08).

       \N is not allowed in a character class. \B, \R, and \X are not special
       inside a character class. Like other unrecognized escape sequences,
       they are treated as the literal characters "B", "R", and "X". Outside a
       character class, these sequences have different meanings.

       Unsupported escape sequences

       In Perl, the sequences \l, \L, \u, and \U are recognized by its string
       handler and used to modify the case of following characters. PCRE does
       not support these escape sequences.

       Absolute and relative back references

       The sequence \g followed by an unsigned or a negative number,
       optionally enclosed in braces, is an absolute or relative back
       reference. A named back reference can be coded as \g{name}. Back
       references are discussed later, following the discussion of
       parenthesized subpatterns.

       Absolute and relative subroutine calls

       For compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for referencing a subpattern as a "subroutine".
       Details are discussed later. Note that \g{...} (Perl syntax) and
       \g<...> (Oniguruma syntax) are not synonymous. The former is a back
       reference; the latter is a subroutine call.

       Generic character types

       Another use of backslash is for specifying generic character types:

         \d:
           any decimal digit

         \D:
           any character that is not a decimal digit

         \h:
           any horizontal white space character

         \H:
           any character that is not a horizontal white space character

         \s:
           any white space character

         \S:
           any character that is not a white space character

         \v:
           any vertical white space character

         \V:
           any character that is not a vertical white space character

         \w:
           any "word" character

         \W:
           any "non-word" character

       There is also the single sequence \N, which matches a non-newline
       character. This is the same as the "." metacharacter when dotall is not
       set. Perl also uses \N to match characters by name; PCRE does not
       support this.

       Each pair of lower and upper case escape sequences partitions the
       complete set of characters into two disjoint sets. Any given character
       matches one, and only one, of each pair. The sequences can appear both
       inside and outside character classes. They each match one character of
       the appropriate type. If the current matching point is at the end of
       the subject string, all of them fail, because there is no character to
       match.

       For compatibility with Perl, \s does not match the VT character (code
       11). This makes it different from the POSIX "space" class. The \s
       characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
       "use locale;" is included in a Perl script, \s may match the VT
       character. In PCRE, it never does.

       A "word" character is an underscore or any character that is a letter
       or digit. By default, the definition of letters and digits is
       controlled by PCRE's low-valued character tables, in Erlang's case (and
       without the unicode option), the ISO-Latin-1 character set.

       By default, in unicode mode, characters with values greater than 255,
       i.e. all characters outside the ISO-Latin-1 character set, never match
       \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
       their original meanings from before UTF support was available, mainly
       for efficiency reasons. However, if the ucp option is set, the
       behaviour is changed so that Unicode properties are used to determine
       character types, as follows:

         \d:
           any character that \p{Nd} matches (decimal digit)

         \s:
           any character that \p{Z} matches, plus HT, LF, FF, CR)

          \w:
           any character that \p{L} or \p{N} matches, plus underscore)

       The upper case escapes match the inverse sets of characters. Note that
       \d matches only decimal digits, whereas \w matches any Unicode digit,
       as well as any Unicode letter, and underscore. Note also that ucp
       affects \b, and \B because they are defined in terms of \w and \W.
       Matching these sequences is noticeably slower when ucp is set.

       The sequences \h, \H, \v, and \V are features that were added to Perl
       at release 5.10. In contrast to the other sequences, which match only
       ASCII characters by default, these always match certain high-valued
       codepoints, whether or not ucp is set. The horizontal space characters
       are:

         U+0009:
           Horizontal tab (HT)

         U+0020:
           Space

         U+00A0:
           Non-break space

         U+1680:
           Ogham space mark

         U+180E:
           Mongolian vowel separator

         U+2000:
           En quad

         U+2001:
           Em quad

         U+2002:
           En space

         U+2003:
           Em space

         U+2004:
           Three-per-em space

         U+2005:
           Four-per-em space

         U+2006:
           Six-per-em space

         U+2007:
           Figure space

         U+2008:
           Punctuation space

         U+2009:
           Thin space

         U+200A:
           Hair space

         U+202F:
           Narrow no-break space

         U+205F:
           Medium mathematical space

         U+3000:
           Ideographic space

       The vertical space characters are:

         U+000A:
           Linefeed (LF)

         U+000B:
           Vertical tab (VT)

         U+000C:
           Form feed (FF)

         U+000D:
           Carriage return (CR)

         U+0085:
           Next line (NEL)

         U+2028:
           Line separator

         U+2029:
           Paragraph separator

       In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
       256 are relevant.

       Newline sequences

       Outside a character class, by default, the escape sequence \R matches
       any Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the
       following:

       (?>\r\n|\n|\x0b|\f|\r|\x85)

       This is an example of an "atomic group", details of which are given
       below.

       This particular group matches either the two-character sequence CR
       followed by LF, or one of the single characters LF (linefeed, U+000A),
       VT (vertical tab, U+000B), FF (form feed, U+000C), CR (carriage return,
       U+000D), or NEL (next line, U+0085). The two-character sequence is
       treated as a single unit that cannot be split.

       In Unicode mode, two additional characters whose codepoints are greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph
       separator, U+2029). Unicode character property support is not needed
       for these characters to be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set of Unicode line endings) by setting the option
       bsr_anycrlf either at compile time or when the pattern is matched. (BSR
       is an abbreviation for "backslash R".) This can be made the default
       when PCRE is built; if this is the case, the other behaviour can be
       requested via the bsr_unicode option. It is also possible to specify
       these settings by starting a pattern string with one of the following
       sequences:

       (*BSR_ANYCRLF) CR, LF, or CRLF only (*BSR_UNICODE) any Unicode newline
       sequence

       These override the default and the options given to the compiling
       function, but they can themselves be overridden by options given to a
       matching function. Note that these special settings, which are not
       Perl-compatible, are recognized only at the very start of a pattern,
       and that they must be in upper case. If more than one of them is
       present, the last one is used. They can be combined with a change of
       newline convention; for example, a pattern can start with:

       (*ANY)(*BSR_ANYCRLF)

       They can also be combined with the (*UTF8), (*UTF) or (*UCP) special
       sequences. Inside a character class, \R is treated as an unrecognized
       escape sequence, and so matches the letter "R" by default.

       Unicode character properties

       Three additional escape sequences that match characters with specific
       properties are available. When in 8-bit non-UTF-8 mode, these sequences
       are of course limited to testing characters whose codepoints are less
       than 256, but they do work in this mode. The extra escape sequences
       are:

         \p{xx}:
           a character with the xx property

         \P{xx}:
           a character without the xx property

         \X:
           a Unicode extended grapheme cluster

       The property names represented by xx above are limited to the Unicode
       script names, the general category properties, "Any", which matches any
       character (including newline), and some special PCRE properties
       (described in the next section). Other Perl properties such as
       "InMusicalSymbols" are not currently supported by PCRE. Note that
       \P{Any} does not match any characters, so always causes a match
       failure.

       Sets of Unicode characters are defined as belonging to certain scripts.
       A character from one of these sets can be matched using a script name.
       For example:

       \p{Greek} \P{Han}

       Those that are not part of an identified script are lumped together as
       "Common". The current list of scripts is:

         * Arabic

         * Armenian

         * Avestan

         * Balinese

         * Bamum

         * Batak

         * Bengali

         * Bopomofo

         * Braille

         * Buginese

         * Buhid

         * Canadian_Aboriginal

         * Carian

         * Chakma

         * Cham

         * Cherokee

         * Common

         * Coptic

         * Cuneiform

         * Cypriot

         * Cyrillic

         * Deseret

         * Devanagari

         * Egyptian_Hieroglyphs

         * Ethiopic

         * Georgian

         * Glagolitic

         * Gothic

         * Greek

         * Gujarati

         * Gurmukhi

         * Han

         * Hangul

         * Hanunoo

         * Hebrew

         * Hiragana

         * Imperial_Aramaic

         * Inherited

         * Inscriptional_Pahlavi

         * Inscriptional_Parthian

         * Javanese

         * Kaithi

         * Kannada

         * Katakana

         * Kayah_Li

         * Kharoshthi

         * Khmer

         * Lao

         * Latin

         * Lepcha

         * Limbu

         * Linear_B

         * Lisu

         * Lycian

         * Lydian

         * Malayalam

         * Mandaic

         * Meetei_Mayek

         * Meroitic_Cursive

         * Meroitic_Hieroglyphs

         * Miao

         * Mongolian

         * Myanmar

         * New_Tai_Lue

         * Nko

         * Ogham

         * Old_Italic

         * Old_Persian

         * Oriya

         * Old_South_Arabian

         * Old_Turkic

         * Ol_Chiki

         * Osmanya

         * Phags_Pa

         * Phoenician

         * Rejang

         * Runic

         * Samaritan

         * Saurashtra

         * Sharada

         * Shavian

         * Sinhala

         * Sora_Sompeng

         * Sundanese

         * Syloti_Nagri

         * Syriac

         * Tagalog

         * Tagbanwa

         * Tai_Le

         * Tai_Tham

         * Tai_Viet

         * Takri

         * Tamil

         * Telugu

         * Thaana

         * Thai

         * Tibetan

         * Tifinagh

         * Ugaritic

         * Vai

         * Yi

       Each character has exactly one Unicode general category property,
       specified by a two-letter abbreviation. For compatibility with Perl,
       negation can be specified by including a circumflex between the opening
       brace and the property name. For example, \p{^Lu} is the same as
       \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the
       general category properties that start with that letter. In this case,
       in the absence of negation, the curly brackets in the escape sequence
       are optional; these two examples have the same effect:

         * \p{L}

         * \pL

       The following general category property codes are supported:

         C:
           Other

         Cc:
           Control

         Cf:
           Format

         Cn:
           Unassigned

         Co:
           Private use

         Cs:
           Surrogate

         L:
           Letter

         Ll:
           Lower case letter

         Lm:
           Modifier letter

         Lo:
           Other letter

         Lt:
           Title case letter

         Lu:
           Upper case letter

         M:
           Mark

         Mc:
           Spacing mark

         Me:
           Enclosing mark

         Mn:
           Non-spacing mark

         N:
           Number

         Nd:
           Decimal number

         Nl:
           Letter number

         No:
           Other number

         P:
           Punctuation

         Pc:
           Connector punctuation

         Pd:
           Dash punctuation

         Pe:
           Close punctuation

         Pf:
           Final punctuation

         Pi:
           Initial punctuation

         Po:
           Other punctuation

         Ps:
           Open punctuation

         S:
           Symbol

         Sc:
           Currency symbol

         Sk:
           Modifier symbol

         Sm:
           Mathematical symbol

         So:
           Other symbol

         Z:
           Separator

         Zl:
           Line separator

         Zp:
           Paragraph separator

         Zs:
           Space separator

       The special property L& is also supported: it matches a character that
       has the Lu, Ll, or Lt property, in other words, a letter that is not
       classified as a modifier or "other".

       The Cs (Surrogate) property applies only to characters in the range
       U+D800 to U+DFFF. Such characters are not valid in Unicode strings and
       so cannot be tested by PCRE. Perl does not support the Cs property

       The long synonyms for property names that Perl supports (such as
       \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned)
       property. Instead, this property is assumed for any code point that is
       not in the Unicode table.

       Specifying caseless matching does not affect these escape sequences.
       For example, \p{Lu} always matches only upper case letters. This is
       different from the behaviour of current versions of Perl.

       Matching characters by Unicode property is not fast, because PCRE has
       to do a multistage table lookup in order to find a character's
       property. That is why the traditional escape sequences such as \d and
       \w do not use Unicode properties in PCRE by default, though you can
       make them do so by setting the ucp option or by starting the pattern
       with (*UCP).

       Extended grapheme clusters

       The \X escape matches any number of Unicode characters that form an
       "extended grapheme cluster", and treats the sequence as an atomic group
       (see below). Up to and including release 8.31, PCRE matched an earlier,
       simpler definition that was equivalent to

       (?>\PM\pM*)

       That is, it matched a character without the "mark" property, followed
       by zero or more characters with the "mark" property. Characters with
       the "mark" property are typically non-spacing accents that affect the
       preceding character.

       This simple definition was extended in Unicode to include more
       complicated kinds of composite character by giving each character a
       grapheme breaking property, and creating rules that use these
       properties to define the boundaries of extended grapheme clusters. In
       releases of PCRE later than 8.31, \X matches one of these clusters.

       \X always matches at least one character. Then it decides whether to
       add additional characters according to the following rules for ending a
       cluster:

         1.:
           End at the end of the subject string.

         2.:
           Do not end between CR and LF; otherwise end after any control
           character.

         3.:
           Do not break Hangul (a Korean script) syllable sequences. Hangul
           characters are of five types: L, V, T, LV, and LVT. An L character
           may be followed by an L, V, LV, or LVT character; an LV or V
           character may be followed by a V or T character; an LVT or T
           character may be follwed only by a T character.

         4.:
           Do not end before extending characters or spacing marks. Characters
           with the "mark" property always have the "extend" grapheme breaking
           property.

         5.:
           Do not end after prepend characters.

         6.:
           Otherwise, end the cluster.

       PCRE's additional properties

       As well as the standard Unicode properties described above, PCRE
       supports four more that make it possible to convert traditional escape
       sequences such as \w and \s and POSIX character classes to use Unicode
       properties. PCRE uses these non-standard, non-Perl properties
       internally when PCRE_UCP is set. However, they may also be used
       explicitly. These properties are:

         Xan:
           Any alphanumeric character

         Xps:
           Any POSIX space character

         Xsp:
           Any Perl space character

         Xwd:
           Any Perl "word" character

       Xan matches characters that have either the L (letter) or the N
       (number) property. Xps matches the characters tab, linefeed, vertical
       tab, form feed, or carriage return, and any other character that has
       the Z (separator) property. Xsp is the same as Xps, except that
       vertical tab is excluded. Xwd matches the same characters as Xan, plus
       underscore.

       There is another non-standard property, Xuc, which matches any
       character that can be represented by a Universal Character Name in C++
       and other programming languages. These are the characters $, @, `
       (grave accent), and all characters with Unicode code points greater
       than or equal to U+00A0, except for the surrogates U+D800 to U+DFFF.
       Note that most base (ASCII) characters are excluded. (Universal
       Character Names are of the form \uHHHH or \UHHHHHHHH where H is a
       hexadecimal digit. Note that the Xuc property does not match these
       sequences but the characters that they represent.)

       Resetting the match start

       The escape sequence \K causes any previously matched characters not to
       be included in the final matched sequence. For example, the pattern:

       foo\Kbar

       matches "foobar", but reports that it has matched "bar". This feature
       is similar to a lookbehind assertion (described below). However, in
       this case, the part of the subject before the real match does not have
       to be of fixed length, as lookbehind assertions do. The use of \K does
       not interfere with the setting of captured substrings. For example,
       when the pattern

       (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       Perl documents that the use of \K within assertions is "not well
       defined". In PCRE, \K is acted upon when it occurs inside positive
       assertions, but is ignored in negative assertions.

       Simple assertions

       The final use of backslash is for certain simple assertions. An
       assertion specifies a condition that has to be met at a particular
       point in a match, without consuming any characters from the subject
       string. The use of subpatterns for more complicated assertions is
       described below. The backslashed assertions are:

         \b:
           matches at a word boundary

         \B:
           matches when not at a word boundary

         \A:
           matches at the start of the subject

         \Z:
           matches at the end of the subject also matches before a newline at
           the end of the subject

         \z:
           matches only at the end of the subject

         \G:
           matches at the first matching position in the subject

       Inside a character class, \b has a different meaning; it matches the
       backspace character. If any other of these assertions appears in a
       character class, by default it matches the corresponding literal
       character (for example, \B matches the letter B).

       A word boundary is a position in the subject string where the current
       character and the previous character do not both match \w or \W (i.e.
       one matches \w and the other matches \W), or the start or end of the
       string if the first or last character matches \w, respectively. In a
       UTF mode, the meanings of \w and \W can be changed by setting the ucp
       option. When this is done, it also affects \b and \B. Neither PCRE nor
       Perl has a separate "start of word" or "end of word" metasequence.
       However, whatever follows \b normally determines which it is. For
       example, the fragment \ba matches "a" at the start of a word.

       The \A, \Z, and \z assertions differ from the traditional circumflex
       and dollar (described in the next section) in that they only ever match
       at the very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These three
       assertions are not affected by the notbol or noteol options, which
       affect only the behaviour of the circumflex and dollar metacharacters.
       However, if the startoffset argument of re:run/3 is non-zero,
       indicating that matching is to start at a point other than the
       beginning of the subject, \A can never match. The difference between \Z
       and \z is that \Z matches before a newline at the end of the string as
       well as at the very end, whereas \z matches only at the end.

       The \G assertion is true only when the current matching position is at
       the start point of the match, as specified by the startoffset argument
       of re:run/3. It differs from \A when the value of startoffset is non-
       zero. By calling re:run/3 multiple times with appropriate arguments,
       you can mimic Perl's /g option, and it is in this kind of
       implementation where \G can be useful.

       Note, however, that PCRE's interpretation of \G, as the start of the
       current match, is subtly different from Perl's, which defines it as the
       end of the previous match. In Perl, these can be different when the
       previously matched string was empty. Because PCRE does just one match
       at a time, it cannot reproduce this behaviour.

       If all the alternatives of a pattern begin with \G, the expression is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.

CIRCUMFLEX AND DOLLAR
       The circumflex and dollar metacharacters are zero-width assertions.
       That is, they test for a particular condition being true without
       consuming any characters from the subject string.

       Outside a character class, in the default matching mode, the circumflex
       character is an assertion that is true only if the current matching
       point is at the start of the subject string. If the startoffset
       argument of re:run/3 is non-zero, circumflex can never match if the
       multiline option is unset. Inside a character class, circumflex has an
       entirely different meaning (see below).

       Circumflex need not be the first character of the pattern if a number
       of alternatives are involved, but it should be the first thing in each
       alternative in which it appears if the pattern is ever to match that
       branch. If all possible alternatives start with a circumflex, that is,
       if the pattern is constrained to match only at the start of the
       subject, it is said to be an "anchored" pattern. (There are also other
       constructs that can cause a pattern to be anchored.)

       The dollar character is an assertion that is true only if the current
       matching point is at the end of the subject string, or immediately
       before a newline at the end of the string (by default). Note, however,
       that it does not actually match the newline. Dollar need not be the
       last character of the pattern if a number of alternatives are involved,
       but it should be the last item in any branch in which it appears.
       Dollar has no special meaning in a character class.

       The meaning of dollar can be changed so that it matches only at the
       very end of the string, by setting the dollar_endonly option at compile
       time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar characters are changed if the
       multiline option is set. When this is the case, a circumflex matches
       immediately after internal newlines as well as at the start of the
       subject string. It does not match after a newline that ends the string.
       A dollar matches before any newlines in the string, as well as at the
       very end, when multiline is set. When newline is specified as the two-
       character sequence CRLF, isolated CR and LF characters do not indicate
       newlines.

       For example, the pattern /^abc$/ matches the subject string "def\nabc"
       (where \n represents a newline) in multiline mode, but not otherwise.
       Consequently, patterns that are anchored in single line mode because
       all branches start with ^ are not anchored in multiline mode, and a
       match for circumflex is possible when the startoffset argument of
       re:run/3 is non-zero. The dollar_endonly option is ignored if multiline
       is set.

       Note that the sequences \A, \Z, and \z can be used to match the start
       and end of the subject in both modes, and if all branches of a pattern
       start with \A it is always anchored, whether or not multiline is set.

FULL STOP (PERIOD, DOT) AND \N
       Outside a character class, a dot in the pattern matches any one
       character in the subject string except (by default) a character that
       signifies the end of a line.

       When a line ending is defined as a single character, dot never matches
       that character; when the two-character sequence CRLF is used, dot does
       not match CR if it is immediately followed by LF, but otherwise it
       matches all characters (including isolated CRs and LFs). When any
       Unicode line endings are being recognized, dot does not match CR or LF
       or any of the other line ending characters.

       The behaviour of dot with regard to newlines can be changed. If the
       dotall option is set, a dot matches any one character, without
       exception. If the two-character sequence CRLF is present in the subject
       string, it takes two dots to match it.

       The handling of dot is entirely independent of the handling of
       circumflex and dollar, the only relationship being that they both
       involve newlines. Dot has no special meaning in a character class.

       The escape sequence \N behaves like a dot, except that it is not
       affected by the PCRE_DOTALL option. In other words, it matches any
       character except one that signifies the end of a line. Perl also uses
       \N to match characters by name; PCRE does not support this.

MATCHING A SINGLE DATA UNIT
       Outside a character class, the escape sequence \C matches any one data
       unit, whether or not a UTF mode is set. One data unit is one byte.
       Unlike a dot, \C always matches line-ending characters. The feature is
       provided in Perl in order to match individual bytes in UTF-8 mode, but
       it is unclear how it can usefully be used. Because \C breaks up
       characters into individual data units, matching one unit with \C in a
       UTF mode means that the rest of the string may start with a malformed
       UTF character. This has undefined results, because PCRE assumes that it
       is dealing with valid UTF strings.

       PCRE does not allow \C to appear in lookbehind assertions (described
       below) in a UTF mode, because this would make it impossible to
       calculate the length of the lookbehind.

       In general, the \C escape sequence is best avoided. However, one way of
       using it that avoids the problem of malformed UTF characters is to use
       a lookahead to check the length of the next character, as in this
       pattern, which could be used with a UTF-8 string (ignore white space
       and line breaks):

         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       A group that starts with (?| resets the capturing parentheses numbers
       in each alternative (see "Duplicate Subpattern Numbers" below). The
       assertions at the start of each branch check the next UTF-8 character
       for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
       character's individual bytes are then captured by the appropriate
       number of groups.

SQUARE BRACKETS AND CHARACTER CLASSES
       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not
       special by default. However, if the PCRE_JAVASCRIPT_COMPAT option is
       set, a lone closing square bracket causes a compile-time error. If a
       closing square bracket is required as a member of the class, it should
       be the first data character in the class (after an initial circumflex,
       if present) or escaped with a backslash.

       A character class matches a single character in the subject. In a UTF
       mode, the character may be more than one data unit long. A matched
       character must be in the set of characters defined by the class, unless
       the first character in the class definition is a circumflex, in which
       case the subject character must not be in the set defined by the class.
       If a circumflex is actually required as a member of the class, ensure
       it is not the first character, or escape it with a backslash.

       For example, the character class [aeiou] matches any lower case vowel,
       while [^aeiou] matches any character that is not a lower case vowel.
       Note that a circumflex is just a convenient notation for specifying the
       characters that are in the class by enumerating those that are not. A
       class that starts with a circumflex is not an assertion; it still
       consumes a character from the subject string, and therefore it fails if
       the current pointer is at the end of the string.

       In UTF-8 mode, characters with values greater than 255 (0xffff) can be
       included in a class as a literal string of data units, or by using the
       \x{ escaping mechanism.

       When caseless matching is set, any letters in a class represent both
       their upper case and lower case versions, so for example, a caseless
       [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
       match "A", whereas a caseful version would. In a UTF mode, PCRE always
       understands the concept of case for characters whose values are less
       than 256, so caseless matching is always possible. For characters with
       higher values, the concept of case is supported if PCRE is compiled
       with Unicode property support, but not otherwise. If you want to use
       caseless matching in a UTF mode for characters 256 and above, you must
       ensure that PCRE is compiled with Unicode property support as well as
       with UTF support.

       Characters that might indicate line breaks are never treated in any
       special way when matching character classes, whatever line-ending
       sequence is in use, and whatever setting of the PCRE_DOTALL and
       PCRE_MULTILINE options is used. A class such as [^a] always matches one
       of these characters.

       The minus (hyphen) character can be used to specify a range of
       characters in a character class. For example, [d-m] matches any letter
       between d and m, inclusive. If a minus character is required in a
       class, it must be escaped with a backslash or appear in a position
       where it cannot be interpreted as indicating a range, typically as the
       first or last character in the class.

       It is not possible to have the literal character "]" as the end
       character of a range. A pattern such as [W-]46] is interpreted as a
       class of two characters ("W" and "-") followed by a literal string
       "46]", so it would match "W46]" or "-46]". However, if the "]" is
       escaped with a backslash it is interpreted as the end of range, so
       [W-\]46] is interpreted as a class containing a range followed by two
       other characters. The octal or hexadecimal representation of "]" can
       also be used to end a range.

       Ranges operate in the collating sequence of character values. They can
       also be used for characters specified numerically, for example
       [\000-\037]. Ranges can include any characters that are valid for the
       current mode.

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if
       character tables for a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases. In UTF modes, PCRE supports the
       concept of case for characters with values greater than 255 only when
       it is compiled with Unicode property support.

       The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
       \w, and \W may appear in a character class, and add the characters that
       they match to the class. For example, [\dABCDEF] matches any
       hexadecimal digit. In UTF modes, the ucp option affects the meanings of
       \d, \s, \w and their upper case partners, just as it does when they
       appear outside a character class, as described in the section entitled
       "Generic character types" above. The escape sequence \b has a different
       meaning inside a character class; it matches the backspace character.
       The sequences \B, \N, \R, and \X are not special inside a character
       class. Like any other unrecognized escape sequences, they are treated
       as the literal characters "B", "N", "R", and "X".

       A circumflex can conveniently be used with the upper case character
       types to specify a more restricted set of characters than the matching
       lower case type. For example, the class [^\W_] matches any letter or
       digit, but not underscore, whereas [\w] includes underscore. A positive
       character class should be read as "something OR something OR ..." and a
       negative class as "NOT something AND NOT something AND NOT ...".

       The only metacharacters that are recognized in character classes are
       backslash, hyphen (only where it can be interpreted as specifying a
       range), circumflex (only at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name - see the
       next section), and the terminating closing square bracket. However,
       escaping other non-alphanumeric characters does no harm.

POSIX CHARACTER CLASSES
       Perl supports the POSIX notation for character classes. This uses names
       enclosed by [: and :] within the enclosing square brackets. PCRE also
       supports this notation. For example,

       [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are:

         alnum:
           letters and digits

         alpha:
           letters

         ascii:
           character codes 0 - 127

         blank:
           space or tab only

         cntrl:
           control characters

         digit:
           decimal digits (same as \d)

         graph:
           printing characters, excluding space

         lower:
           lower case letters

         print:
           printing characters, including space

         punct:
           printing characters, excluding letters and digits and space

         space:
           whitespace (not quite the same as \s)

         upper:
           upper case letters

         word:
           "word" characters (same as \w)

         xdigit:
           hexadecimal digits

       The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
       and space (32). Notice that this list includes the VT character (code
       11). This makes "space" different to \s, which does not include VT (for
       Perl compatibility).

       The name "word" is a Perl extension, and "blank" is a GNU extension
       from Perl 5.8. Another Perl extension is negation, which is indicated
       by a ^ character after the colon. For example,

       [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       By default, in UTF modes, characters with values greater than 255 do
       not match any of the POSIX character classes. However, if the PCRE_UCP
       option is passed to pcre_compile() , some of the classes are changed so
       that Unicode character properties are used. This is achieved by
       replacing the POSIX classes by other sequences, as follows:

         [:alnum:]:
           becomes \p{Xan}

         [:alpha:]:
           becomes \p{L}

         [:blank:]:
           becomes \h

         [:digit::
           becomes \p{Nd}

         [:lower:]:
           becomes \p{Ll}

         [:space:]:
           becomes \p{Xps}

         [:upper::
           becomes \p{Lu}

         [:word:]:
           becomes \p{Xwd}

       Negated versions, such as [:^alpha:] use \P instead of \p. The other
       POSIX classes are unchanged, and match only characters with code points
       less than 256.

VERTICAL BAR
       Vertical bar characters are used to separate alternative patterns. For
       example, the pattern

       gilbert|sullivan

       matches either "gilbert" or "sullivan". Any number of alternatives may
       appear, and an empty alternative is permitted (matching the empty
       string). The matching process tries each alternative in turn, from left
       to right, and the first one that succeeds is used. If the alternatives
       are within a subpattern (defined below), "succeeds" means matching the
       rest of the main pattern as well as the alternative in the subpattern.

INTERNAL OPTION SETTING
       The settings of the caseless, multiline, dotall, and extended options
       (which are Perl-compatible) can be changed from within the pattern by a
       sequence of Perl option letters enclosed between "(?" and ")". The
       option letters are

         i:
           for caseless

         m:
           for multiline

         s:
           for dotall

         x:
           for extended

       For example, (?im) sets caseless, multiline matching. It is also
       possible to unset these options by preceding the letter with a hyphen,
       and a combined setting and unsetting such as (?im-sx), which sets
       caseless and multiline while unsetting dotall and extended, is also
       permitted. If a letter appears both before and after the hyphen, the
       option is unset.

       The PCRE-specific options dupnames, ungreedy, and extra can be changed
       in the same way as the Perl-compatible options by using the characters
       J, U and X respectively.

       When one of these option changes occurs at top level (that is, not
       inside subpattern parentheses), the change applies to the remainder of
       the pattern that follows. If the change is placed right at the start of
       a pattern, PCRE extracts it into the global options.

       An option change within a subpattern (see below for a description of
       subpatterns) affects only that part of the subpattern that follows it,
       so

       (a(?i)b)c

       matches abc and aBc and no other strings (assuming caseless is not
       used). By this means, options can be made to have different settings in
       different parts of the pattern. Any changes made in one alternative do
       carry on into subsequent branches within the same subpattern. For
       example,

       (a(?i)b|c)

       matches "ab", "aB", "c", and "C", even though when matching "C" the
       first branch is abandoned before the option setting. This is because
       the effects of option settings happen at compile time. There would be
       some very weird behaviour otherwise.

       Note: There are other PCRE-specific options that can be set by the
       application when the compiling or matching functions are called. In
       some cases the pattern can contain special leading sequences such as
       (*CRLF) to override what the application has set or what has been
       defaulted. Details are given in the section entitled "Newline
       sequences" above. There are also the (*UTF8) and (*UCP) leading
       sequences that can be used to set UTF and Unicode property modes; they
       are equivalent to setting the unicode and the ucp options,
       respectively. The (*UTF) sequence is a generic version that can be used
       with any of the libraries. However, the application can set the
       never_utf option, which locks out the use of the (*UTF) sequences.

SUBPATTERNS
       Subpatterns are delimited by parentheses (round brackets), which can be
       nested. Turning part of a pattern into a subpattern does two things:

       1. It localizes a set of alternatives. For example, the pattern

       cat(aract|erpillar|)

       matches "cataract", "caterpillar", or "cat". Without the parentheses,
       it would match "cataract", "erpillar" or an empty string.

       2. It sets up the subpattern as a capturing subpattern. This means
       that, when the complete pattern matches, that portion of the subject
       string that matched the subpattern is passed back to the caller via the
       return value of re:run/3.

       Opening parentheses are counted from left to right (starting from 1) to
       obtain numbers for the capturing subpatterns.For example, if the string
       "the red king" is matched against the pattern

       the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are
       numbered 1, 2, and 3, respectively.

       The fact that plain parentheses fulfil two functions is not always
       helpful. There are often times when a grouping subpattern is required
       without a capturing requirement. If an opening parenthesis is followed
       by a question mark and a colon, the subpattern does not do any
       capturing, and is not counted when computing the number of any
       subsequent capturing subpatterns. For example, if the string "the white
       queen" is matched against the pattern

       the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capturing subpatterns is 65535.

       As a convenient shorthand, if any option settings are required at the
       start of a non-capturing subpattern, the option letters may appear
       between the "?" and the ":". Thus the two patterns

         * (?i:saturday|sunday)

         * (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried from left to right, and options are not reset until the end of
       the subpattern is reached, an option setting in one branch does affect
       subsequent branches, so the above patterns match "SUNDAY" as well as
       "Saturday".

DUPLICATE SUBPATTERN NUMBERS
       Perl 5.10 introduced a feature whereby each alternative in a subpattern
       uses the same numbers for its capturing parentheses. Such a subpattern
       starts with (?| and is itself a non-capturing subpattern. For example,
       consider this pattern:

       (?|(Sat)ur|(Sun))day

       Because the two alternatives are inside a (?| group, both sets of
       capturing parentheses are numbered one. Thus, when the pattern matches,
       you can look at captured substring number one, whichever alternative
       matched. This construct is useful when you want to capture part, but
       not all, of one of a number of alternatives. Inside a (?| group,
       parentheses are numbered as usual, but the number is reset at the start
       of each branch. The numbers of any capturing parentheses that follow
       the subpattern start after the highest number used in any branch. The
       following example is taken from the Perl documentation. The numbers
       underneath show in which buffer the captured content will be stored.

         # before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

       A back reference to a numbered subpattern uses the most recent value
       that is set for that number by any subpattern. The following pattern
       matches "abcabc" or "defdef":

       /(?|(abc)|(def))\1/

       In contrast, a subroutine call to a numbered subpattern always refers
       to the first one in the pattern with the given number. The following
       pattern matches "abcabc" or "defabc":

       /(?|(abc)|(def))(?1)/

       If a condition test for a subpattern's having matched refers to a non-
       unique number, the test is true if any of the subpatterns of that
       number have matched.

       An alternative approach to using this "branch reset" feature is to use
       duplicate named subpatterns, as described in the next section.

NAMED SUBPATTERNS
       Identifying capturing parentheses by number is simple, but it can be
       very hard to keep track of the numbers in complicated regular
       expressions. Furthermore, if an expression is modified, the numbers may
       change. To help with this difficulty, PCRE supports the naming of
       subpatterns. This feature was not added to Perl until release 5.10.
       Python had the feature earlier, and PCRE introduced it at release 4.0,
       using the Python syntax. PCRE now supports both the Perl and the Python
       syntax. Perl allows identically numbered subpatterns to have different
       names, but PCRE does not.

       In PCRE, a subpattern can be named in one of three ways: (?<name>...)
       or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
       to capturing parentheses from other parts of the pattern, such as back
       references, recursion, and conditions, can be made by name as well as
       by number.

       Names consist of up to 32 alphanumeric characters and underscores.
       Named capturing parentheses are still allocated numbers as well as
       names, exactly as if the names were not present. The capture
       specification to re:run/3 can use named values if they are present in
       the regular expression.

       By default, a name must be unique within a pattern, but it is possible
       to relax this constraint by setting the dupnames option at compile
       time. (Duplicate names are also always permitted for subpatterns with
       the same number, set up as described in the previous section.)
       Duplicate names can be useful for patterns where only one instance of
       the named parentheses can match. Suppose you want to match the name of
       a weekday, either as a 3-letter abbreviation or as the full name, and
       in both cases you want to extract the abbreviation. This pattern
       (ignoring the line breaks) does the job:

         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

       There are five capturing substrings, but only one is ever set after a
       match. (An alternative way of solving this problem is to use a "branch
       reset" subpattern, as described in the previous section.)

       In case of capturing named subpatterns which names are not unique, the
       first matching occurrence (counted from left to right in the subject)
       is returned from re:exec/3, if the name is specified in the values part
       of the capture statement. The all_names capturing value will match all
       of the names in the same way.

       Warning: You cannot use different names to distinguish between two
       subpatterns with the same number because PCRE uses only the numbers
       when matching. For this reason, an error is given at compile time if
       different names are given to subpatterns with the same number. However,
       you can give the same name to subpatterns with the same number, even
       when dupnames is not set.

REPETITION
       Repetition is specified by quantifiers, which can follow any of the
       following items:

         * a literal data character

         * the dot metacharacter

         * the \C escape sequence

         * the \X escape sequence

         * the \R escape sequence

         * an escape such as \d or \pL that matches a single character

         * a character class

         * a back reference (see next section)

         * a parenthesized subpattern (including assertions)

         * a subroutine call to a subpattern (recursive or otherwise)

       The general repetition quantifier specifies a minimum and maximum
       number of permitted matches, by giving the two numbers in curly
       brackets (braces), separated by a comma. The numbers must be less than
       65536, and the first must be less than or equal to the second. For
       example:

       z{2,4}

       matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
       special character. If the second number is omitted, but the comma is
       present, there is no upper limit; if the second number and the comma
       are both omitted, the quantifier specifies an exact number of required
       matches. Thus

       [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, while

       \d{8}

       matches exactly 8 digits. An opening curly bracket that appears in a
       position where a quantifier is not allowed, or one that does not match
       the syntax of a quantifier, is taken as a literal character. For
       example, {,6} is not a quantifier, but a literal string of four
       characters.

       In Unicode mode, quantifiers apply to characters rather than to
       individual data units. Thus, for example, \x{100}{2} matches two
       characters, each of which is represented by a two-byte sequence in a
       UTF-8 string. Similarly, \X{3} matches three Unicode extended grapheme
       clusters, each of which may be several data units long (and they may be
       of different lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be
       useful for subpatterns that are referenced as subroutines from
       elsewhere in the pattern (but see also the section entitled "Defining
       subpatterns for use by reference only" below). Items other than
       subpatterns that have a {0} quantifier are omitted from the compiled
       pattern.

       For convenience, the three most common quantifiers have single-
       character abbreviations:

         *:
           is equivalent to {0,}

         +:
           is equivalent to {1,}

         ?:
           is equivalent to {0,1}

       It is possible to construct infinite loops by following a subpattern
       that can match no characters with a quantifier that has no upper limit,
       for example:

       (a?)*

       Earlier versions of Perl and PCRE used to give an error at compile time
       for such patterns. However, because there are cases where this can be
       useful, such patterns are now accepted, but if any repetition of the
       subpattern does in fact match no characters, the loop is forcibly
       broken.

       By default, the quantifiers are "greedy", that is, they match as much
       as possible (up to the maximum number of permitted times), without
       causing the rest of the pattern to fail. The classic example of where
       this gives problems is in trying to match comments in C programs. These
       appear between /* and */ and within the comment, individual * and /
       characters may appear. An attempt to match C comments by applying the
       pattern

       /\*.*\*/

       to the string

       /* first comment */ not comment /* second comment */

       fails, because it matches the entire string owing to the greediness of
       the .* item.

       However, if a quantifier is followed by a question mark, it ceases to
       be greedy, and instead matches the minimum number of times possible, so
       the pattern

       /\*.*?\*/

       does the right thing with the C comments. The meaning of the various
       quantifiers is not otherwise changed, just the preferred number of
       matches. Do not confuse this use of question mark with its use as a
       quantifier in its own right. Because it has two uses, it can sometimes
       appear doubled, as in

       \d??\d

       which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

       If the ungreedy option is set (an option that is not available in
       Perl), the quantifiers are not greedy by default, but individual ones
       can be made greedy by following them with a question mark. In other
       words, it inverts the default behaviour.

       When a parenthesized subpattern is quantified with a minimum repeat
       count that is greater than 1 or with a limited maximum, more memory is
       required for the compiled pattern, in proportion to the size of the
       minimum or maximum.

       If a pattern starts with .* or .{0,} and the dotall option (equivalent
       to Perl's /s) is set, thus allowing the dot to match newlines, the
       pattern is implicitly anchored, because whatever follows will be tried
       against every character position in the subject string, so there is no
       point in retrying the overall match at any position after the first.
       PCRE normally treats such a pattern as though it were preceded by \A.

       In cases where it is known that the subject string contains no
       newlines, it is worth setting dotall in order to obtain this
       optimization, or alternatively using ^ to indicate anchoring
       explicitly.

       However, there are some cases where the optimization cannot be used.
       When .* is inside capturing parentheses that are the subject of a back
       reference elsewhere in the pattern, a match at the start may fail where
       a later one succeeds. Consider, for example:

       (.*)abc\1

       If the subject is "xyz123abc123" the match point is the fourth
       character. For this reason, such a pattern is not implicitly anchored.

       Another case where implicit anchoring is not applied is when the
       leading .* is inside an atomic group. Once again, a match at the start
       may fail where a later one succeeds. Consider this pattern:

       (?>.*?a)b

       It matches "ab" in the subject "aab". The use of the backtracking
       control verbs (*PRUNE) and (*SKIP) also disable this optimization.

       When a capturing subpattern is repeated, the value captured is the
       substring that matched the final iteration. For example, after

       (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of the captured substring
       is "tweedledee". However, if there are nested capturing subpatterns,
       the corresponding captured values may have been set in previous
       iterations. For example, after

       /(a|(b))+/

       matches "aba" the value of the second captured substring is "b".

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
       With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
       repetition, failure of what follows normally causes the repeated item
       to be re-evaluated to see if a different number of repeats allows the
       rest of the pattern to match. Sometimes it is useful to prevent this,
       either to change the nature of the match, or to cause it fail earlier
       than it otherwise might, when the author of the pattern knows there is
       no point in carrying on.

       Consider, for example, the pattern \d+foo when applied to the subject
       line

       123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits matching the
       \d+ item, and then with 4, and so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
       the means for specifying that once a subpattern has matched, it is not
       to be re-evaluated in this way.

       If we use atomic grouping for the previous example, the matcher gives
       up immediately on failing to match "foo" the first time. The notation
       is a kind of special parenthesis, starting with (?> as in this example:

       (?>\d+)foo

       This kind of parenthesis "locks up" the part of the pattern it contains
       once it has matched, and a failure further into the pattern is
       prevented from backtracking into it. Backtracking past it to previous
       items, however, works as normal.

       An alternative description is that a subpattern of this type matches
       the string of characters that an identical standalone pattern would
       match, if anchored at the current point in the subject string.

       Atomic grouping subpatterns are not capturing subpatterns. Simple cases
       such as the above example can be thought of as a maximizing repeat that
       must swallow everything it can. So, while both \d+ and \d+? are
       prepared to adjust the number of digits they match in order to make the
       rest of the pattern match, (?>\d+) can only match an entire sequence of
       digits.

       Atomic groups in general can of course contain arbitrarily complicated
       subpatterns, and can be nested. However, when the subpattern for an
       atomic group is just a single repeated item, as in the example above, a
       simpler notation, called a "possessive quantifier" can be used. This
       consists of an additional + character following a quantifier. Using
       this notation, the previous example can be rewritten as

       \d++foo

       Note that a possessive quantifier can be used with an entire group, for
       example:

       (abc|xyz){2,3}+

       Possessive quantifiers are always greedy; the setting of the ungreedy
       option is ignored. They are a convenient notation for the simpler forms
       of atomic group. However, there is no difference in the meaning of a
       possessive quantifier and the equivalent atomic group, though there may
       be a performance difference; possessive quantifiers should be slightly
       faster.

       The possessive quantifier syntax is an extension to the Perl 5.8
       syntax. Jeffrey Friedl originated the idea (and the name) in the first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built Sun's Java package, and PCRE copied it from there. It ultimately
       found its way into Perl at release 5.10.

       PCRE has an optimization that automatically "possessifies" certain
       simple pattern constructs. For example, the sequence A+B is treated as
       A++B because there is no point in backtracking into a sequence of A's
       when B must follow.

       When a pattern contains an unlimited repeat inside a subpattern that
       can itself be repeated an unlimited number of times, the use of an
       atomic group is the only way to avoid some failing matches taking a
       very long time indeed. The pattern

       (\D+|<\d+>)*[!?]

       matches an unlimited number of substrings that either consist of non-
       digits, or digits enclosed in <>, followed by either ! or ?. When it
       matches, it runs quickly. However, if it is applied to

       aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it takes a long time before reporting failure. This is because the
       string can be divided between the internal \D+ repeat and the external
       * repeat in a large number of ways, and all have to be tried. (The
       example uses [!?] rather than a single character at the end, because
       both PCRE and Perl have an optimization that allows for fast failure
       when a single character is used. They remember the last single
       character that is required for a match, and fail early if it is not
       present in the string.) If the pattern is changed so that it uses an
       atomic group, like this:

       ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.

BACK REFERENCES
       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a capturing
       subpattern earlier (that is, to its left) in the pattern, provided
       there have been that many previous capturing left parentheses.

       However, if the decimal number following the backslash is less than 10,
       it is always taken as a back reference, and causes an error only if
       there are not that many capturing left parentheses in the entire
       pattern. In other words, the parentheses that are referenced need not
       be to the left of the reference for numbers less than 10. A "forward
       back reference" of this type can make sense when a repetition is
       involved and the subpattern to the right has participated in an earlier
       iteration.

       It is not possible to have a numerical "forward back reference" to a
       subpattern whose number is 10 or more using this syntax because a
       sequence such as \50 is interpreted as a character defined in octal.
       See the subsection entitled "Non-printing characters" above for further
       details of the handling of digits following a backslash. There is no
       such problem when named parentheses are used. A back reference to any
       subpattern is possible using named parentheses (see below).

       Another way of avoiding the ambiguity inherent in the use of digits
       following a backslash is to use the \g escape sequence. This escape
       must be followed by an unsigned number or a negative number, optionally
       enclosed in braces. These examples are all identical:

         * (ring), \1

         * (ring), \g1

         * (ring), \g{1}

       An unsigned number specifies an absolute reference without the
       ambiguity that is present in the older syntax. It is also useful when
       literal digits follow the reference. A negative number is a relative
       reference. Consider this example:

       (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the most recently started
       capturing subpattern before \g, that is, is it equivalent to \2 in this
       example. Similarly, \g{-2} would be equivalent to \1. The use of
       relative references can be helpful in long patterns, and also in
       patterns that are created by joining together fragments that contain
       references within themselves.

       A back reference matches whatever actually matched the capturing
       subpattern in the current subject string, rather than anything matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing that). So the pattern

       (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If caseful matching is in force at the
       time of the back reference, the case of letters is relevant. For
       example,

       ((?i)rah)\s+\1

       matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
       original capturing subpattern is matched caselessly.

       There are several different ways of writing back references to named
       subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
       \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
       unified back reference syntax, in which \g can be used for both numeric
       and named references, is also supported. We could rewrite the above
       example in any of the following ways:

         * (?<p1>(?i)rah)\s+\k<p1>

         * (?'p1'(?i)rah)\s+\k{p1}

         * (?P<p1>(?i)rah)\s+(?P=p1)

         * (?<p1>(?i)rah)\s+\g{p1}

       A subpattern that is referenced by name may appear in the pattern
       before or after the reference.

       There may be more than one back reference to the same subpattern. If a
       subpattern has not actually been used in a particular match, any back
       references to it always fail. For example, the pattern

       (a|(bc))\2

       always fails if it starts to match "a" rather than "bc". Because there
       may be many capturing parentheses in a pattern, all digits following
       the backslash are taken as part of a potential back reference number.
       If the pattern continues with a digit character, some delimiter must be
       used to terminate the back reference. If the extended option is set,
       this can be whitespace. Otherwise an empty comment (see "Comments"
       below) can be used.

       Recursive back references

       A back reference that occurs inside the parentheses to which it refers
       fails when the subpattern is first used, so, for example, (a\1) never
       matches. However, such references can be useful inside repeated
       subpatterns. For example, the pattern

       (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each
       iteration of the subpattern, the back reference matches the character
       string corresponding to the previous iteration. In order for this to
       work, the pattern must be such that the first iteration does not need
       to match the back reference. This can be done using alternation, as in
       the example above, or by a quantifier with a minimum of zero.

       Back references of this type cause the group that they reference to be
       treated as an atomic group. Once the whole group has been matched, a
       subsequent matching failure cannot cause backtracking into the middle
       of the group.

ASSERTIONS
       An assertion is a test on the characters following or preceding the
       current matching point that does not actually consume any characters.
       The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
       described above.

       More complicated assertions are coded as subpatterns. There are two
       kinds: those that look ahead of the current position in the subject
       string, and those that look behind it. An assertion subpattern is
       matched in the normal way, except that it does not cause the current
       matching position to be changed.

       Assertion subpatterns are not capturing subpatterns. If such an
       assertion contains capturing subpatterns within it, these are counted
       for the purposes of numbering the capturing subpatterns in the whole
       pattern. However, substring capturing is carried out only for positive
       assertions. (Perl sometimes, but not always, does do capturing in
       negative assertions.)

       For compatibility with Perl, assertion subpatterns may be repeated;
       though it makes no sense to assert the same thing several times, the
       side effect of capturing parentheses may occasionally be useful. In
       practice, there only three cases:

         (1):
           If the quantifier is {0}, the assertion is never obeyed during
           matching. However, it may contain internal capturing parenthesized
           groups that are called from elsewhere via the subroutine mechanism.

         (2):
           If quantifier is {0,n} where n is greater than zero, it is treated
           as if it were {0,1}. At run time, the rest of the pattern match is
           tried with and without the assertion, the order depending on the
           greediness of the quantifier.

         (3):
           If the minimum repetition is greater than zero, the quantifier is
           ignored. The assertion is obeyed just once when encountered during
           matching.

       Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

       \w+(?=;)

       matches a word followed by a semicolon, but does not include the
       semicolon in the match, and

       foo(?!bar)

       matches any occurrence of "foo" that is not followed by "bar". Note
       that the apparently similar pattern

       (?!foo)bar

       does not find an occurrence of "bar" that is preceded by something
       other than "foo"; it finds any occurrence of "bar" whatsoever, because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most convenient way to do it is with (?!) because an empty string
       always matches, so an assertion that requires there not to be an empty
       string must always fail. The backtracking control verb (*FAIL) or (*F)
       is a synonym for (?!).

       Lookbehind assertions

       Lookbehind assertions start with (?<= for positive assertions and (?<!
       for negative assertions. For example,

       (?<!foo)bar

       does find an occurrence of "bar" that is not preceded by "foo". The
       contents of a lookbehind assertion are restricted such that all the
       strings it matches must have a fixed length. However, if there are
       several top-level alternatives, they do not all have to have the same
       fixed length. Thus

       (?<=bullock|donkey)

       is permitted, but

       (?<!dogs?|cats?)

       causes an error at compile time. Branches that match different length
       strings are permitted only at the top level of a lookbehind assertion.
       This is an extension compared with Perl, which requires all branches to
       match the same length of string. An assertion such as

       (?<=ab(c|de))

       is not permitted, because its single top-level branch can match two
       different lengths, but it is acceptable to PCRE if rewritten to use two
       top-level branches:

       (?<=abc|abde)

       In some cases, the escape sequence \K (see above) can be used instead
       of a lookbehind assertion to get round the fixed-length restriction.

       The implementation of lookbehind assertions is, for each alternative,
       to temporarily move the current position back by the fixed length and
       then try to match. If there are insufficient characters before the
       current position, the assertion fails.

       In a UTF mode, PCRE does not allow the \C escape (which matches a
       single data unit even in a UTF mode) to appear in lookbehind
       assertions, because it makes it impossible to calculate the length of
       the lookbehind. The \X and \R escapes, which can match different
       numbers of data units, are also not permitted.

       "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
       lookbehinds, as long as the subpattern matches a fixed-length string.
       Recursion, however, is not supported.

       Possessive quantifiers can be used in conjunction with lookbehind
       assertions to specify efficient matching of fixed-length strings at the
       end of subject strings. Consider a simple pattern such as

       abcd$

       when applied to a long string that does not match. Because matching
       proceeds from left to right, PCRE will look for each "a" in the subject
       and then see if what follows matches the rest of the pattern. If the
       pattern is specified as

       ^.*abcd$

       the initial .* matches the entire string at first, but when this fails
       (because there is no following "a"), it backtracks to match all but the
       last character, then all but the last two characters, and so on. Once
       again the search for "a" covers the entire string, from right to left,
       so we are no better off. However, if the pattern is written as

       ^.*+(?<=abcd)

       there can be no backtracking for the .*+ item; it can match only the
       entire string. The subsequent lookbehind assertion does a single test
       on the last four characters. If it fails, the match fails immediately.
       For long strings, this approach makes a significant difference to the
       processing time.

       Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

       (?<=\d{3})(?<!999)foo

       matches "foo" preceded by three digits that are not "999". Notice that
       each of the assertions is applied independently at the same point in
       the subject string. First there is a check that the previous three
       characters are all digits, and then there is a check that the same
       three characters are not "999". This pattern does not match "foo"
       preceded by six characters, the first of which are digits and the last
       three of which are not "999". For example, it doesn't match
       "123abcfoo". A pattern to do that is

       (?<=\d{3}...)(?<!999)foo

       This time the first assertion looks at the preceding six characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

       (?<=(?<!foo)bar)baz

       matches an occurrence of "baz" that is preceded by "bar" which in turn
       is not preceded by "foo", while

       (?<=\d{3}(?!999)...)foo

       is another pattern that matches "foo" preceded by three digits and any
       three characters that are not "999".

CONDITIONAL SUBPATTERNS
       It is possible to cause the matching process to obey a subpattern
       conditionally or to choose between two alternative subpatterns,
       depending on the result of an assertion, or whether a specific
       capturing subpattern has already been matched. The two possible forms
       of conditional subpattern are:

         * (?(condition)yes-pattern)

         * (?(condition)yes-pattern|no-pattern)

       If the condition is satisfied, the yes-pattern is used; otherwise the
       no-pattern (if present) is used. If there are more than two
       alternatives in the subpattern, a compile-time error occurs. Each of
       the two alternatives may itself contain nested subpatterns of any form,
       including conditional subpatterns; the restriction to two alternatives
       applies only at the level of the condition. This pattern fragment is an
       example where the alternatives are complex:

       (?(1) (A|B|C) | (D | (?(2)E|F) | E) )

       There are four kinds of condition: references to subpatterns,
       references to recursion, a pseudo-condition called DEFINE, and
       assertions.

       Checking for a used subpattern by number

       If the text between the parentheses consists of a sequence of digits,
       the condition is true if a capturing subpattern of that number has
       previously matched. If there is more than one capturing subpattern with
       the same number (see the earlier section about duplicate subpattern
       numbers), the condition is true if any of them have matched. An
       alternative notation is to precede the digits with a plus or minus
       sign. In this case, the subpattern number is relative rather than
       absolute. The most recently opened parentheses can be referenced by
       (?(-1), the next most recent by (?(-2), and so on. Inside loops it can
       also make sense to refer to subsequent groups. The next parentheses to
       be opened can be referenced as (?(+1), and so on. (The value zero in
       any of these forms is not used; it provokes a compile-time error.)

       Consider the following pattern, which contains non-significant
       whitespace to make it more readable (assume the extended option) and to
       divide it into three parts for ease of discussion:

       ( \( )? [^()]+ (?(1) \) )

       The first part matches an optional opening parenthesis, and if that
       character is present, sets it as the first captured substring. The
       second part matches one or more characters that are not parentheses.
       The third part is a conditional subpattern that tests whether or not
       the first set of parentheses matched or not. If they did, that is, if
       subject started with an opening parenthesis, the condition is true, and
       so the yes-pattern is executed and a closing parenthesis is required.
       Otherwise, since no-pattern is not present, the subpattern matches
       nothing. In other words, this pattern matches a sequence of non-
       parentheses, optionally enclosed in parentheses.

       If you were embedding this pattern in a larger one, you could use a
       relative reference:

       ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...

       This makes the fragment independent of the parentheses in the larger
       pattern.

       Checking for a used subpattern by name

       Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
       used subpattern by name. For compatibility with earlier versions of
       PCRE, which had this facility before Perl, the syntax (?(name)...) is
       also recognized. However, there is a possible ambiguity with this
       syntax, because subpattern names may consist entirely of digits. PCRE
       looks first for a named subpattern; if it cannot find one and the name
       consists entirely of digits, PCRE looks for a subpattern of that
       number, which must be greater than zero. Using subpattern names that
       consist entirely of digits is not recommended.

       Rewriting the above example to use a named subpattern gives this:

       (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )

       If the name used in a condition of this kind is a duplicate, the test
       is applied to all subpatterns of the same name, and is true if any one
       of them has matched.

       Checking for pattern recursion

       If the condition is the string (R), and there is no subpattern with the
       name R, the condition is true if a recursive call to the whole pattern
       or any subpattern has been made. If digits or a name preceded by
       ampersand follow the letter R, for example:

       (?(R3)...) or (?(R&name)...)

       the condition is true if the most recent recursion is into a subpattern
       whose number or name is given. This condition does not check the entire
       recursion stack. If the name used in a condition of this kind is a
       duplicate, the test is applied to all subpatterns of the same name, and
       is true if any one of them is the most recent recursion.

       At "top level", all these recursion test conditions are false. The
       syntax for recursive patterns is described below.

       Defining subpatterns for use by reference only

       If the condition is the string (DEFINE), and there is no subpattern
       with the name DEFINE, the condition is always false. In this case,
       there may be only one alternative in the subpattern. It is always
       skipped if control reaches this point in the pattern; the idea of
       DEFINE is that it can be used to define "subroutines" that can be
       referenced from elsewhere. (The use of subroutines is described below.)
       For example, a pattern to match an IPv4 address such as
       "192.168.23.245" could be written like this (ignore whitespace and line
       breaks):

       (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) \b
       (?&byte) (\.(?&byte)){3} \b

       The first part of the pattern is a DEFINE group inside which a another
       group named "byte" is defined. This matches an individual component of
       an IPv4 address (a number less than 256). When matching takes place,
       this part of the pattern is skipped because DEFINE acts like a false
       condition. The rest of the pattern uses references to the named group
       to match the four dot-separated components of an IPv4 address,
       insisting on a word boundary at each end.

       Assertion conditions

       If the condition is not in any of the above formats, it must be an
       assertion. This may be a positive or negative lookahead or lookbehind
       assertion. Consider this pattern, again containing non-significant
       whitespace, and with the two alternatives on the second line:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The condition is a positive lookahead assertion that matches an
       optional sequence of non-letters followed by a letter. In other words,
       it tests for the presence of at least one letter in the subject. If a
       letter is found, the subject is matched against the first alternative;
       otherwise it is matched against the second. This pattern matches
       strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
       letters and dd are digits.

COMMENTS
       There are two ways of including comments in patterns that are processed
       by PCRE. In both cases, the start of the comment must not be in a
       character class, nor in the middle of any other sequence of related
       characters such as (?: or a subpattern name or number. The characters
       that make up a comment play no part in the pattern matching.

       The sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses are not permitted. If the
       PCRE_EXTENDED option is set, an unescaped # character also introduces a
       comment, which in this case continues to immediately after the next
       newline character or character sequence in the pattern. Which
       characters are interpreted as newlines is controlled by the options
       passed to a compiling function or by a special sequence at the start of
       the pattern, as described in the section entitled "Newline conventions"
       above. Note that the end of this type of comment is a literal newline
       sequence in the pattern; escape sequences that happen to represent a
       newline do not count. For example, consider this pattern when extended
       is set, and the default newline convention is in force:

       abc #comment \n still comment

       On encountering the # character, pcre_compile()  skips along, looking
       for a newline in the pattern. The sequence \n is still literal at this
       stage, so it does not terminate the comment. Only an actual character
       with the code value 0x0a (the default newline) does so.

RECURSIVE PATTERNS
       Consider the problem of matching a string in parentheses, allowing for
       unlimited nested parentheses. Without the use of recursion, the best
       that can be done is to use a pattern that matches up to some fixed
       depth of nesting. It is not possible to handle an arbitrary nesting
       depth.

       For some time, Perl has provided a facility that allows regular
       expressions to recurse (amongst other things). It does this by
       interpolating Perl code in the expression at run time, and the code can
       refer to the expression itself. A Perl pattern using code interpolation
       to solve the parentheses problem can be created like this:

       $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears.

       Obviously, PCRE cannot support the interpolation of Perl code. Instead,
       it supports special syntax for recursion of the entire pattern, and
       also for individual subpattern recursion. After its introduction in
       PCRE and Python, this kind of recursion was subsequently introduced
       into Perl at release 5.10.

       A special item that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive subroutine call of the
       subpattern of the given number, provided that it occurs inside that
       subpattern. (If not, it is a non-recursive subroutine call, which is
       described in the next section.) The special item (?R) or (?0) is a
       recursive call of the entire regular expression.

       This PCRE pattern solves the nested parentheses problem (assume the
       extended option is set so that whitespace is ignored):

       \( ( [^()]++ | (?R) )* \)

       First it matches an opening parenthesis. Then it matches any number of
       substrings which can either be a sequence of non-parentheses, or a
       recursive match of the pattern itself (that is, a correctly
       parenthesized substring). Finally there is a closing parenthesis. Note
       the use of a possessive quantifier to avoid backtracking into sequences
       of non-parentheses.

       If this were part of a larger pattern, you would not want to recurse
       the entire pattern, so instead you could use this:

       ( \( ( [^()]++ | (?1) )* \) )

       We have put the pattern into parentheses, and caused the recursion to
       refer to them instead of the whole pattern.

       In a larger pattern, keeping track of parenthesis numbers can be
       tricky. This is made easier by the use of relative references. Instead
       of (?1) in the pattern above you can write (?-2) to refer to the second
       most recently opened parentheses preceding the recursion. In other
       words, a negative number counts capturing parentheses leftwards from
       the point at which it is encountered.

       It is also possible to refer to subsequently opened parentheses, by
       writing references such as (?+2). However, these cannot be recursive
       because the reference is not inside the parentheses that are
       referenced. They are always non-recursive subroutine calls, as
       described in the next section.

       An alternative approach is to use named parentheses instead. The Perl
       syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
       supported. We could rewrite the above example as follows:

       (?<pn> \( ( [^()]++ | (?&pn) )* \) )

       If there is more than one subpattern with the same name, the earliest
       one is used.

       This particular example pattern that we have been looking at contains
       nested unlimited repeats, and so the use of a possessive quantifier for
       matching strings of non-parentheses is important when applying the
       pattern to strings that do not match. For example, when this pattern is
       applied to

       (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it yields "no match" quickly. However, if a possessive quantifier is
       not used, the match runs for a very long time indeed because there are
       so many different ways the + and * repeats can carve up the subject,
       and all have to be tested before failure can be reported.

       At the end of a match, the values of capturing parentheses are those
       from the outermost level. If the pattern above is matched against

       (ab(cd)ef)

       the value for the inner capturing parentheses (numbered 2) is "ef",
       which is the last value taken on at the top level. If a capturing
       subpattern is not matched at the top level, its final captured value is
       unset, even if it was (temporarily) set at a deeper level during the
       matching process.

       Do not confuse the (?R) item with the condition (R), which tests for
       recursion. Consider this pattern, which matches text in angle brackets,
       allowing for arbitrary nesting. Only digits are allowed in nested
       brackets (that is, when recursing), whereas any characters are
       permitted at the outer level.

       < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >

       In this pattern, (?(R) is the start of a conditional subpattern, with
       two different alternatives for the recursive and non-recursive cases.
       The (?R) item is the actual recursive call.

       Differences in recursion processing between PCRE and Perl

       Recursion processing in PCRE differs from Perl in two important ways.
       In PCRE (like Python, but unlike Perl), a recursive subpattern call is
       always treated as an atomic group. That is, once it has matched some of
       the subject string, it is never re-entered, even if it contains untried
       alternatives and there is a subsequent matching failure. This can be
       illustrated by the following pattern, which purports to match a
       palindromic string that contains an odd number of characters (for
       example, "a", "aba", "abcba", "abcdcba"):

       ^(.|(.)(?1)\2)$

       The idea is that it either matches a single character, or two identical
       characters surrounding a sub-palindrome. In Perl, this pattern works;
       in PCRE it does not if the pattern is longer than three characters.
       Consider the subject string "abcba":

       At the top level, the first character is matched, but as it is not at
       the end of the string, the first alternative fails; the second
       alternative is taken and the recursion kicks in. The recursive call to
       subpattern 1 successfully matches the next character ("b"). (Note that
       the beginning and end of line tests are not part of the recursion).

       Back at the top level, the next character ("c") is compared with what
       subpattern 2 matched, which was "a". This fails. Because the recursion
       is treated as an atomic group, there are now no backtracking points,
       and so the entire match fails. (Perl is able, at this point, to re-
       enter the recursion and try the second alternative.) However, if the
       pattern is written with the alternatives in the other order, things are
       different:

       ^((.)(?1)\2|.)$

       This time, the recursing alternative is tried first, and continues to
       recurse until it runs out of characters, at which point the recursion
       fails. But this time we do have another alternative to try at the
       higher level. That is the big difference: in the previous case the
       remaining alternative is at a deeper recursion level, which PCRE cannot
       use.

       To change the pattern so that it matches all palindromic strings, not
       just those with an odd number of characters, it is tempting to change
       the pattern to this:

       ^((.)(?1)\2|.?)$

       Again, this works in Perl, but not in PCRE, and for the same reason.
       When a deeper recursion has matched a single character, it cannot be
       entered again in order to match an empty string. The solution is to
       separate the two cases, and write out the odd and even cases as
       alternatives at the higher level:

       ^(?:((.)(?1)\2|)|((.)(?3)\4|.))

       If you want to match typical palindromic phrases, the pattern has to
       ignore all non-word characters, which can be done like this:

       ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$

       If run with the caseless option, this pattern matches phrases such as
       "A man, a plan, a canal: Panama!" and it works well in both PCRE and
       Perl. Note the use of the possessive quantifier *+ to avoid
       backtracking into sequences of non-word characters. Without this, PCRE
       takes a great deal longer (ten times or more) to match typical phrases,
       and Perl takes so long that you think it has gone into a loop.

       WARNING: The palindrome-matching patterns above work only if the
       subject string does not start with a palindrome that is shorter than
       the entire string. For example, although "abcba" is correctly matched,
       if the subject is "ababa", PCRE finds the palindrome "aba" at the
       start, then fails at top level because the end of the string does not
       follow. Once again, it cannot jump back into the recursion to try other
       alternatives, so the entire match fails.

       The second way in which PCRE and Perl differ in their recursion
       processing is in the handling of captured values. In Perl, when a
       subpattern is called recursively or as a subpattern (see the next
       section), it has no access to any values that were captured outside the
       recursion, whereas in PCRE these values can be referenced. Consider
       this pattern:

       ^(.)(\1|a(?2))

       In PCRE, this pattern matches "bab". The first capturing parentheses
       match "b", then in the second group, when the back reference \1 fails
       to match "b", the second alternative matches "a" and then recurses. In
       the recursion, \1 does now match "b" and so the whole match succeeds.
       In Perl, the pattern fails to match because inside the recursive call
       \1 cannot access the externally set value.

SUBPATTERNS AS SUBROUTINES
       If the syntax for a recursive subpattern call (either by number or by
       name) is used outside the parentheses to which it refers, it operates
       like a subroutine in a programming language. The called subpattern may
       be defined before or after the reference. A numbered reference can be
       absolute or relative, as in these examples:

         * (...(absolute)...)...(?2)...

         * (...(relative)...)...(?-1)...

         * (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

       (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If instead the pattern

       (sens|respons)e and (?1)ibility

       is used, it does match "sense and responsibility" as well as the other
       two strings. Another example is given in the discussion of DEFINE
       above.

       All subroutine calls, whether recursive or not, are always treated as
       atomic groups. That is, once a subroutine has matched some of the
       subject string, it is never re-entered, even if it contains untried
       alternatives and there is a subsequent matching failure. Any capturing
       parentheses that are set during the subroutine call revert to their
       previous values afterwards.

       Processing options such as case-independence are fixed when a
       subpattern is defined, so if it is used as a subroutine, such options
       cannot be changed for different calls. For example, consider this
       pattern:

       (abc)(?i:(?-1))

       It matches "abcabc". It does not match "abcABC" because the change of
       processing option does not affect the called subpattern.

ONIGURUMA SUBROUTINE SYNTAX
       For compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for referencing a subpattern as a subroutine,
       possibly recursively. Here are two of the examples used above,
       rewritten using this syntax:

       (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )

       (sens|respons)e and \g'1'ibility

       PCRE supports an extension to Oniguruma: if a number is preceded by a
       plus or a minus sign it is taken as a relative reference. For example:

       (abc)(?i:\g<-1>)

       Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
       synonymous. The former is a back reference; the latter is a subroutine
       call.

BACKTRACKING CONTROL
       Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
       which are still described in the Perl documentation as "experimental
       and subject to change or removal in a future version of Perl". It goes
       on to say: "Their usage in production code should be noted to avoid
       problems during upgrades." The same remarks apply to the PCRE features
       described in this section.

       The new verbs make use of what was previously invalid syntax: an
       opening parenthesis followed by an asterisk. They are generally of the
       form (*VERB) or (*VERB:NAME). Some may take either form, possibly
       behaving differently depending on whether or not a name is present. A
       name is any sequence of characters that does not include a closing
       parenthesis. The maximum length of name is 255 in the 8-bit library and
       65535 in the 16-bit and 32-bit libraries. If the name is empty, that
       is, if the closing parenthesis immediately follows the colon, the
       effect is as if the colon were not there. Any number of these verbs may
       occur in a pattern.

       The behaviour of these verbs in repeated groups, assertions, and in
       subpatterns called as subroutines (whether or not recursively) is
       documented below.

       Optimizations that affect backtracking verbs

       PCRE contains some optimizations that are used to speed up matching by
       running some checks at the start of each match attempt. For example, it
       may know the minimum length of matching subject, or that a particular
       character must be present. When one of these optimizations bypasses the
       running of a match, any included backtracking verbs will not, of
       course, be processed. You can suppress the start-of-match optimizations
       by setting the no_start_optimize option when calling re:compile/2 or
       re:run/3, or by starting the pattern with (*NO_START_OPT).

       Experiments with Perl suggest that it too has similar optimizations,
       sometimes leading to anomalous results.

       Verbs that act immediately

       The following verbs act as soon as they are encountered. They may not
       be followed by a name.

       (*ACCEPT)

       This verb causes the match to end successfully, skipping the remainder
       of the pattern. However, when it is inside a subpattern that is called
       as a subroutine, only that subpattern is ended successfully. Matching
       then continues at the outer level. If (*ACCEPT) in triggered in a
       positive assertion, the assertion succeeds; in a negative assertion,
       the assertion fails.

       If (*ACCEPT) is inside capturing parentheses, the data so far is
       captured. For example:

       A((?:A|B(*ACCEPT)|C)D)

       This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is
       captured by the outer parentheses.

       (*FAIL) or (*F)

       This verb causes a matching failure, forcing backtracking to occur. It
       is equivalent to (?!) but easier to read. The Perl documentation notes
       that it is probably useful only when combined with (?{}) or (??{}).
       Those are, of course, Perl features that are not present in PCRE. The
       nearest equivalent is the callout feature, as for example in this
       pattern:

       a+(?C)(*FAIL)

       A match with the string "aaaa" always fails, but the callout is taken
       before each backtrack happens (in this example, 10 times).

       Recording which path was taken

       There is one verb whose main purpose is to track how a match was
       arrived at, though it also has a secondary use in conjunction with
       advancing the match starting point (see (*SKIP) below).

   Warning:
       In Erlang, there is no interface to retrieve a mark with re:run/{2,3],
       so only the secondary purpose is relevant to the Erlang programmer!

       The rest of this section is therefore deliberately not adapted for
       reading by the Erlang programmer, however the examples might help in
       understanding NAMES as they can be used by (*SKIP).

       (*MARK:NAME) or (*:NAME)

       A name is always required with this verb. There may be as many
       instances of (*MARK) as you like in a pattern, and their names do not
       have to be unique.

       When a match succeeds, the name of the last-encountered (*MARK:NAME),
       (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to
       the caller as described in the section entitled "Extra data for
       pcre_exec()" in the pcreapi documentation. Here is an example of
       pcretest output, where the /K modifier requests the retrieval and
       outputting of (*MARK) data:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/K
         data> XY
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this
       example it indicates which of the two alternatives matched. This is a
       more efficient way of obtaining this information than putting each
       alternative in its own capturing parentheses.

       If a verb with a name is encountered in a positive assertion that is
       true, the name is recorded and passed back if it is the last-
       encountered. This does not happen for negative assertions or failing
       positive assertions.

       After a partial match or a failed match, the last encountered name in
       the entire match process is returned. For example:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/K
         data> XP
         No match, mark = B

       Note that in this unanchored example the mark is retained from the
       match attempt that started at the letter "X" in the subject. Subsequent
       match attempts starting at "P" and then with an empty string do not get
       as far as the (*MARK) item, but nevertheless do not reset it.

       Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching
       continues with what follows, but if there is no subsequent match,
       causing a backtrack to the verb, a failure is forced. That is,
       backtracking cannot pass to the left of the verb. However, when one of
       these verbs appears inside an atomic group or an assertion that is
       true, its effect is confined to that group, because once the group has
       been matched, there is never any backtracking into it. In this
       situation, backtracking can "jump back" to the left of the entire
       atomic group or assertion. (Remember also, as stated above, that this
       localization also applies in subroutine calls.)

       These verbs differ in exactly what kind of failure occurs when
       backtracking reaches them. The behaviour described below is what
       happens when the verb is not in a subroutine or an assertion.
       Subsequent sections cover these special cases.

       (*COMMIT)

       This verb, which may not be followed by a name, causes the whole match
       to fail outright if there is a later matching failure that causes
       backtracking to reach it. Even if the pattern is unanchored, no further
       attempts to find a match by advancing the starting point take place. If
       (*COMMIT) is the only backtracking verb that is encountered, once it
       has been passed re:run/{2,3} is committed to finding a match at the
       current starting point, or not at all. For example:

       a+(*COMMIT)b

       This matches "xxaab" but not "aacaab". It can be thought of as a kind
       of dynamic anchor, or "I've started, so I must finish." The name of the
       most recently passed (*MARK) in the path is passed back when (*COMMIT)
       forces a match failure.

       If there is more than one backtracking verb in a pattern, a different
       one that follows (*COMMIT) may be triggered first, so merely passing
       (*COMMIT) during a match does not always guarantee that a match must be
       at this starting point.

       Note that (*COMMIT) at the start of a pattern is not the same as an
       anchor, unless PCRE's start-of-match optimizations are turned off, as
       shown in this example:

         1> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list}]).
         {match,["abc"]}
         2> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list},no_start_optimize]).
         nomatch

       PCRE knows that any match must start with "a", so the optimization
       skips along the subject to "a" before running the first match attempt,
       which succeeds. When the optimization is disabled by the
       no_start_optimize option, the match starts at "x" and so the (*COMMIT)
       causes it to fail without trying any other starting points.

       (*PRUNE) or (*PRUNE:NAME)

       This verb causes the match to fail at the current starting position in
       the subject if there is a later matching failure that causes
       backtracking to reach it. If the pattern is unanchored, the normal
       "bumpalong" advance to the next starting character then happens.
       Backtracking can occur as usual to the left of (*PRUNE), before it is
       reached, or when matching to the right of (*PRUNE), but if there is no
       match to the right, backtracking cannot cross (*PRUNE). In simple
       cases, the use of (*PRUNE) is just an alternative to an atomic group or
       possessive quantifier, but there are some uses of (*PRUNE) that cannot
       be expressed in any other way. In an anchored pattern (*PRUNE) has the
       same effect as (*COMMIT).

       The behaviour of (*PRUNE:NAME) is the not the same as
       (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is
       remembered for passing back to the caller. However, (*SKIP:NAME)
       searches only for names set with (*MARK).

   Warning:
       The fact that (*PRUNE:NAME) remembers the name is useless to the Erlang
       programmer, as names can not be retrieved.

       (*SKIP)

       This verb, when given without a name, is like (*PRUNE), except that if
       the pattern is unanchored, the "bumpalong" advance is not to the next
       character, but to the position in the subject where (*SKIP) was
       encountered. (*SKIP) signifies that whatever text was matched leading
       up to it cannot be part of a successful match. Consider:

       a+(*SKIP)b

       If the subject is "aaaac...", after the first match attempt fails
       (starting at the first character in the string), the starting point
       skips on to start the next attempt at "c". Note that a possessive
       quantifer does not have the same effect as this example; although it
       would suppress backtracking during the first match attempt, the second
       attempt would start at the second character instead of skipping on to
       "c".

       (*SKIP:NAME)

       When (*SKIP) has an associated name, its behaviour is modified. When it
       is triggered, the previous path through the pattern is searched for the
       most recent (*MARK) that has the same name. If one is found, the
       "bumpalong" advance is to the subject position that corresponds to that
       (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with
       a matching name is found, the (*SKIP) is ignored.

       Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
       ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME).

       (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next innermost alternative when
       backtracking reaches it. That is, it cancels any further backtracking
       within the current alternative. Its name comes from the observation
       that it can be used for a pattern-based if-then-else block:

       ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If the COND1 pattern matches, FOO is tried (and possibly further items
       after the end of the group if FOO succeeds); on failure, the matcher
       skips to the second alternative and tries COND2, without backtracking
       into COND1. If that succeeds and BAR fails, COND3 is tried. If
       subsequently BAZ fails, there are no more alternatives, so there is a
       backtrack to whatever came before the entire group. If (*THEN) is not
       inside an alternation, it acts like (*PRUNE).

       The behaviour of (*THEN:NAME) is the not the same as
       (*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is
       remembered for passing back to the caller. However, (*SKIP:NAME)
       searches only for names set with (*MARK).

   Warning:
       The fact that (*THEN:NAME) remembers the name is useless to the Erlang
       programmer, as names can not be retrieved.

       A subpattern that does not contain a | character is just a part of the
       enclosing alternative; it is not a nested alternation with only one
       alternative. The effect of (*THEN) extends beyond such a subpattern to
       the enclosing alternative. Consider this pattern, where A, B, etc. are
       complex pattern fragments that do not contain any | characters at this
       level:

       A (B(*THEN)C) | D

       If A and B are matched, but there is a failure in C, matching does not
       backtrack into A; instead it moves to the next alternative, that is, D.
       However, if the subpattern containing (*THEN) is given an alternative,
       it behaves differently:

       A (B(*THEN)C | (*FAIL)) | D

       The effect of (*THEN) is now confined to the inner subpattern. After a
       failure in C, matching moves to (*FAIL), which causes the whole
       subpattern to fail because there are no more alternatives to try. In
       this case, matching does now backtrack into A.

       Note that a conditional subpattern is not considered as having two
       alternatives, because only one is ever used. In other words, the |
       character in a conditional subpattern has a different meaning. Ignoring
       white space, consider:

       ^.*? (?(?=a) a | b(*THEN)c )

       If the subject is "ba", this pattern does not match. Because .*? is
       ungreedy, it initially matches zero characters. The condition (?=a)
       then fails, the character "b" is matched, but "c" is not. At this
       point, matching does not backtrack to .*? as might perhaps be expected
       from the presence of the | character. The conditional subpattern is
       part of the single alternative that comprises the whole pattern, and so
       the match fails. (If there was a backtrack into .*?, allowing it to
       match "b", the match would succeed.)

       The verbs just described provide four different "strengths" of control
       when subsequent matching fails. (*THEN) is the weakest, carrying on the
       match at the next alternative. (*PRUNE) comes next, failing the match
       at the current starting position, but allowing an advance to the next
       character (for an unanchored pattern). (*SKIP) is similar, except that
       the advance may be more than one character. (*COMMIT) is the strongest,
       causing the entire match to fail.

       More than one backtracking verb

       If more than one backtracking verb is present in a pattern, the one
       that is backtracked onto first acts. For example, consider this
       pattern, where A, B, etc. are complex pattern fragments:

       (A(*COMMIT)B(*THEN)C|ABD)

       If A matches but B fails, the backtrack to (*COMMIT) causes the entire
       match to fail. However, if A and B match, but C fails, the backtrack to
       (*THEN) causes the next alternative (ABD) to be tried. This behaviour
       is consistent, but is not always the same as Perl's. It means that if
       two or more backtracking verbs appear in succession, all the the last
       of them has no effect. Consider this example:

       ...(*COMMIT)(*PRUNE)...

       If there is a matching failure to the right, backtracking onto (*PRUNE)
       cases it to be triggered, and its action is taken. There can never be a
       backtrack onto (*COMMIT).

       Backtracking verbs in repeated groups

       PCRE differs from Perl in its handling of backtracking verbs in
       repeated groups. For example, consider:

       /(a(*COMMIT)b)+ac/

       If the subject is "abac", Perl matches, but PCRE fails because the
       (*COMMIT) in the second repeat of the group acts.

       Backtracking verbs in assertions

       (*FAIL) in an assertion has its normal effect: it forces an immediate
       backtrack.

       (*ACCEPT) in a positive assertion causes the assertion to succeed
       without any further processing. In a negative assertion, (*ACCEPT)
       causes the assertion to fail without any further processing.

       The other backtracking verbs are not treated specially if they appear
       in a positive assertion. In particular, (*THEN) skips to the next
       alternative in the innermost enclosing group that has alternations,
       whether or not this is within the assertion.

       Negative assertions are, however, different, in order to ensure that
       changing a positive assertion into a negative assertion changes its
       result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a
       negative assertion to be true, without considering any further
       alternative branches in the assertion. Backtracking into (*THEN) causes
       it to skip to the next enclosing alternative within the assertion (the
       normal behaviour), but if the assertion does not have such an
       alternative, (*THEN) behaves like (*PRUNE).

       Backtracking verbs in subroutines

       These behaviours occur whether or not the subpattern is called
       recursively. Perl's treatment of subroutines is different in some
       cases.

       (*FAIL) in a subpattern called as a subroutine has its normal effect:
       it forces an immediate backtrack.

       (*ACCEPT) in a subpattern called as a subroutine causes the subroutine
       match to succeed without any further processing. Matching then
       continues after the subroutine call.

       (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine
       cause the subroutine match to fail.

       (*THEN) skips to the next alternative in the innermost enclosing group
       within the subpattern that has alternatives. If there is no such group
       within the subpattern, (*THEN) causes the subroutine match to fail.

Ericsson AB                       stdlib 2.4                             re(3)

NAME | DESCRIPTION | DATA TYPES | EXPORTS | PERL LIKE REGULAR EXPRESSIONS SYNTAX | PCRE REGULAR EXPRESSION DETAILS | SPECIAL START-OF-PATTERN ITEMS | CHARACTERS AND METACHARACTERS | BACKSLASH | CIRCUMFLEX AND DOLLAR | FULL STOP (PERIOD, DOT) AND \N | MATCHING A SINGLE DATA UNIT | SQUARE BRACKETS AND CHARACTER CLASSES | POSIX CHARACTER CLASSES | VERTICAL BAR | INTERNAL OPTION SETTING | SUBPATTERNS | DUPLICATE SUBPATTERN NUMBERS | NAMED SUBPATTERNS | REPETITION | ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS | BACK REFERENCES | ASSERTIONS | CONDITIONAL SUBPATTERNS | COMMENTS | RECURSIVE PATTERNS | SUBPATTERNS AS SUBROUTINES | ONIGURUMA SUBROUTINE SYNTAX | BACKTRACKING CONTROL

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