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PERLRE(1)	       Perl Programmers	Reference Guide		     PERLRE(1)

       perlre -	Perl regular expressions

       This page describes the syntax of regular expressions in	Perl.

       If you haven't used regular expressions before, a quick-start introduc-
       tion is available in perlrequick, and a longer tutorial introduction is
       available in perlretut.

       For reference on	how regular expressions	are used in matching opera-
       tions, plus various examples of the same, see discussions of "m//",
       "s///", "qr//" and "??" in "Regexp Quote-Like Operators"	in perlop.


       Matching	operations can have various modifiers.	Modifiers that relate
       to the interpretation of	the regular expression inside are listed be-
       low.  Modifiers that alter the way a regular expression is used by Perl
       are detailed in "Regexp Quote-Like Operators" in	perlop and "Gory de-
       tails of	parsing	quoted constructs" in perlop.

       m   Treat string	as multiple lines.  That is, change "^"	and "$"	from
	   matching the	start or end of	the string to matching the start or
	   end of any line anywhere within the string.

       s   Treat string	as single line.	 That is, change "." to	match any
	   character whatsoever, even a	newline, which normally	it would not

	   The "/s" and	"/m" modifiers both override the $* setting.  That is,
	   no matter what $* contains, "/s" without "/m" will force "^"	to
	   match only at the beginning of the string and "$" to	match only at
	   the end (or just before a newline at	the end) of the	string.	 To-
	   gether, as /ms, they	let the	"." match any character	whatsoever,
	   while still allowing	"^" and	"$" to match, respectively, just after
	   and just before newlines within the string.

       i   Do case-insensitive pattern matching.

	   If "use locale" is in effect, the case map is taken from the	cur-
	   rent	locale.	 See perllocale.

       x   Extend your pattern's legibility by permitting whitespace and com-

       g and c
	   Global matching, and	keep the Current position after	failed match-
	   ing.	 Unlike	i, m, s	and x, these two flags affect the way the
	   regex is used rather	than the regex itself. See "Using regular ex-
	   pressions in	Perl" in perlretut for further explanation of the g
	   and c modifiers.

       These are usually written as "the "/x" modifier", even though the de-
       limiter in question might not really be a slash.	 Any of	these modi-
       fiers may also be embedded within the regular expression	itself using
       the "(?...)" construct.	See below.

       The "/x"	modifier itself	needs a	little more explanation.  It tells the
       regular expression parser to ignore whitespace that is neither back-
       slashed nor within a character class.  You can use this to break	up
       your regular expression into (slightly) more readable parts.  The "#"
       character is also treated as a metacharacter introducing	a comment,
       just as in ordinary Perl	code.  This also means that if you want	real
       whitespace or "#" characters in the pattern (outside a character	class,
       where they are unaffected by "/x"), then	you'll either have to escape
       them (using backslashes or "\Q...\E") or	encode them using octal	or hex
       escapes.	 Taken together, these features	go a long way towards making
       Perl's regular expressions more readable.  Note that you	have to	be
       careful not to include the pattern delimiter in the comment--perl has
       no way of knowing you did not intend to close the pattern early.	 See
       the C-comment deletion code in perlop.  Also note that anything inside
       a "\Q...\E" stays unaffected by "/x".

       Regular Expressions

       The patterns used in Perl pattern matching evolved from those supplied
       in the Version 8	regex routines.	 (The routines are derived (distantly)
       from Henry Spencer's freely redistributable reimplementation of the V8
       routines.)  See "Version	8 Regular Expressions" for details.

       In particular the following metacharacters have their standard
       egrep-ish meanings:

	   \   Quote the next metacharacter
	   ^   Match the beginning of the line
	   .   Match any character (except newline)
	   $   Match the end of	the line (or before newline at the end)
	   |   Alternation
	   ()  Grouping
	   []  Character class

       By default, the "^" character is	guaranteed to match only the beginning
       of the string, the "$" character	only the end (or before	the newline at
       the end), and Perl does certain optimizations with the assumption that
       the string contains only	one line.  Embedded newlines will not be
       matched by "^" or "$".  You may,	however, wish to treat a string	as a
       multi-line buffer, such that the	"^" will match after any newline
       within the string (except if the	newline	is the last character in the
       string),	and "$"	will match before any newline.	At the cost of a lit-
       tle more	overhead, you can do this by using the /m modifier on the pat-
       tern match operator.  (Older programs did this by setting $*, but this
       practice	is now deprecated.)

       To simplify multi-line substitutions, the "." character never matches a
       newline unless you use the "/s" modifier, which in effect tells Perl to
       pretend the string is a single line--even if it isn't.  The "/s"	modi-
       fier also overrides the setting of $*, in case you have some (badly be-
       haved) older code that sets it in another module.

       The following standard quantifiers are recognized:

	   *	  Match	0 or more times
	   +	  Match	1 or more times
	   ?	  Match	1 or 0 times
	   {n}	  Match	exactly	n times
	   {n,}	  Match	at least n times
	   {n,m}  Match	at least n but not more	than m times

       (If a curly bracket occurs in any other context,	it is treated as a
       regular character.  In particular, the lower bound is not optional.)
       The "*" quantifier is equivalent	to "{0,}", the "+" quantifier to
       "{1,}", and the "?" quantifier to "{0,1}".  n and m are limited to in-
       tegral values less than a preset	limit defined when perl	is built.
       This is usually 32766 on	the most common	platforms.  The	actual limit
       can be seen in the error	message	generated by code such as this:

	   $_ **= $_ , / {$_} /	for 2 .. 42;

       By default, a quantified	subpattern is "greedy",	that is, it will match
       as many times as	possible (given	a particular starting location)	while
       still allowing the rest of the pattern to match.	 If you	want it	to
       match the minimum number	of times possible, follow the quantifier with
       a "?".  Note that the meanings don't change, just the "greediness":

	   *?	  Match	0 or more times, not greedily
	   +?	  Match	1 or more times, not greedily
	   ??	  Match	0 or 1 time, not greedily
	   {n}?	  Match	exactly	n times, not greedily
	   {n,}?  Match	at least n times, not greedily
	   {n,m}? Match	at least n but not more	than m times, not greedily

       Because patterns	are processed as double	quoted strings,	the following
       also work:

	   \t	       tab		     (HT, TAB)
	   \n	       newline		     (LF, NL)
	   \r	       return		     (CR)
	   \f	       form feed	     (FF)
	   \a	       alarm (bell)	     (BEL)
	   \e	       escape (think troff)  (ESC)
	   \033	       octal char	     (example: ESC)
	   \x1B	       hex char		     (example: ESC)
	   \x{263a}    long hex	char	     (example: Unicode SMILEY)
	   \cK	       control char	     (example: VT)
	   \N{name}    named Unicode character
	   \l	       lowercase next char (think vi)
	   \u	       uppercase next char (think vi)
	   \L	       lowercase till \E (think	vi)
	   \U	       uppercase till \E (think	vi)
	   \E	       end case	modification (think vi)
	   \Q	       quote (disable) pattern metacharacters till \E

       If "use locale" is in effect, the case map used by "\l",	"\L", "\u" and
       "\U" is taken from the current locale.  See perllocale.	For documenta-
       tion of "\N{name}", see charnames.

       You cannot include a literal "$"	or "@" within a	"\Q" sequence.	An un-
       escaped "$" or "@" interpolates the corresponding variable, while es-
       caping will cause the literal string "\$" to be matched.	 You'll	need
       to write	something like "m/\Quser\E\@\Qhost/".

       In addition, Perl defines the following:

	   \w  Match a "word" character	(alphanumeric plus "_")
	   \W  Match a non-"word" character
	   \s  Match a whitespace character
	   \S  Match a non-whitespace character
	   \d  Match a digit character
	   \D  Match a non-digit character
	   \pP Match P,	named property.	 Use \p{Prop} for longer names.
	   \PP Match non-P
	   \X  Match eXtended Unicode "combining character sequence",
	       equivalent to (?>\PM\pM*)
	   \C  Match a single C	char (octet) even under	Unicode.
	       NOTE: breaks up characters into their UTF-8 bytes,
	       so you may end up with malformed	pieces of UTF-8.
	       Unsupported in lookbehind.

       A "\w" matches a	single alphanumeric character (an alphabetic charac-
       ter, or a decimal digit)	or "_",	not a whole word.  Use "\w+" to	match
       a string	of Perl-identifier characters (which isn't the same as match-
       ing an English word).  If "use locale" is in effect, the	list of	alpha-
       betic characters	generated by "\w" is taken from	the current locale.
       See perllocale.	You may	use "\w", "\W",	"\s", "\S", "\d", and "\D"
       within character	classes, but they aren't usable	as either end of a
       range. If any of	them precedes or follows a "-",	the "-"	is understood
       literally. If Unicode is	in effect, "\s"	matches	also "\x{85}",
       "\x{2028}, and "\x{2029}". See perlunicode for more details about
       "\pP", "\PP", "\X" and the possibility of defining your own "\p"	and
       "\P" properties,	and perluniintro about Unicode in general.

       The POSIX character class syntax


       is also available.  Note	that the "[" and "]" brackets are literal;
       they must always	be used	within a character class expression.

	   # this is correct:
	   $string =~ /[[:alpha:]]/;

	   # this is not, and will generate a warning:
	   $string =~ /[:alpha:]/;

       The available classes and their backslash equivalents (if available)
       are as follows:

	   blank	       [1]
	   digit       \d
	   space       \s      [2]
	   word	       \w      [3]

       [1] A GNU extension equivalent to "[ \t]", "all horizontal whitespace".

       [2] Not exactly equivalent to "\s" since	the "[[:space:]]" includes
	   also	the (very rare)	"vertical tabulator", "\cK" or chr(11) in

       [3] A Perl extension, see above.

       For example use "[:upper:]" to match all	the uppercase characters.
       Note that the "[]" are part of the "[::]" construct, not	part of	the
       whole character class.  For example:


       matches zero, one, any alphabetic character, and	the percent sign.

       The following equivalences to Unicode \p{} constructs and equivalent
       backslash character classes (if available), will	hold:

	   [[:...:]]   \p{...}	       backslash

	   alpha       IsAlpha
	   alnum       IsAlnum
	   ascii       IsASCII
	   cntrl       IsCntrl
	   digit       IsDigit	      \d
	   graph       IsGraph
	   lower       IsLower
	   print       IsPrint
	   punct       IsPunct
	   space       IsSpace
		       IsSpacePerl    \s
	   upper       IsUpper
	   word	       IsWord
	   xdigit      IsXDigit

       For example "[[:lower:]]" and "\p{IsLower}" are equivalent.

       If the "utf8" pragma is not used	but the	"locale" pragma	is, the
       classes correlate with the usual	isalpha(3) interface (except for
       "word" and "blank").

       The other named classes are:

	   Any control character.  Usually characters that don't produce out-
	   put as such but instead control the terminal	somehow: for example
	   newline and backspace are control characters.  All characters with
	   ord() less than 32 are usually classified as	control	characters
	   (assuming ASCII, the	ISO Latin character sets, and Unicode),	as is
	   the character with the ord()	value of 127 ("DEL").

	   Any alphanumeric or punctuation (special) character.

	   Any alphanumeric or punctuation (special) character or the space

	   Any punctuation (special) character.

	   Any hexadecimal digit.  Though this may feel	silly ([0-9A-Fa-f]
	   would work just fine) it is included	for completeness.

       You can negate the [::] character classes by prefixing the class	name
       with a '^'. This	is a Perl extension.  For example:

	   POSIX	 traditional  Unicode

	   [[:^digit:]]	   \D	      \P{IsDigit}
	   [[:^space:]]	   \S	      \P{IsSpace}
	   [[:^word:]]	   \W	      \P{IsWord}

       Perl respects the POSIX standard	in that	POSIX character	classes	are
       only supported within a character class.	 The POSIX character classes
       [.cc.] and [=cc=] are recognized	but not	supported and trying to	use
       them will cause an error.

       Perl defines the	following zero-width assertions:

	   \b  Match a word boundary
	   \B  Match except at a word boundary
	   \A  Match only at beginning of string
	   \Z  Match only at end of string, or before newline at the end
	   \z  Match only at end of string
	   \G  Match only at pos() (e.g. at the	end-of-match position
	       of prior	m//g)

       A word boundary ("\b") is a spot	between	two characters that has	a "\w"
       on one side of it and a "\W" on the other side of it (in	either order),
       counting	the imaginary characters off the beginning and end of the
       string as matching a "\W".  (Within character classes "\b" represents
       backspace rather	than a word boundary, just as it normally does in any
       double-quoted string.)  The "\A"	and "\Z" are just like "^" and "$",
       except that they	won't match multiple times when	the "/m" modifier is
       used, while "^" and "$" will match at every internal line boundary.  To
       match the actual	end of the string and not ignore an optional trailing
       newline,	use "\z".

       The "\G"	assertion can be used to chain global matches (using "m//g"),
       as described in "Regexp Quote-Like Operators" in	perlop.	 It is also
       useful when writing "lex"-like scanners,	when you have several patterns
       that you	want to	match against consequent substrings of your string,
       see the previous	reference.  The	actual location	where "\G" will	match
       can also	be influenced by using "pos()" as an lvalue: see "pos" in
       perlfunc. Currently "\G"	is only	fully supported	when anchored to the
       start of	the pattern; while it is permitted to use it elsewhere,	as in
       "/(?<=\G..)./g",	some such uses ("/.\G/g", for example) currently cause
       problems, and it	is recommended that you	avoid such usage for now.

       The bracketing construct	"( ... )" creates capture buffers. To refer to
       the current contents of a buffer	later on, within the same pattern, use
       \1 for the first, \2 for	the second, and	so on.	Outside	the match use
       "$" instead of "\".  (The \<digit> notation works in certain circum-
       stances outside the match.  See the warning below about \1 vs $1	for
       details.)  Referring back to another part of the	match is called	a

       There is	no limit to the	number of captured substrings that you may
       use.  However Perl also uses \10, \11, etc. as aliases for \010,	\011,
       etc.  (Recall that 0 means octal, so \011 is the	character at number 9
       in your coded character set; which would	be the 10th character, a hori-
       zontal tab under	ASCII.)	 Perl resolves this ambiguity by interpreting
       \10 as a	backreference only if at least 10 left parentheses have	opened
       before it.  Likewise \11	is a backreference only	if at least 11 left
       parentheses have	opened before it.  And so on.  \1 through \9 are al-
       ways interpreted	as backreferences.


	   s/^([^ ]*) *([^ ]*)/$2 $1/;	   # swap first	two words

	    if (/(.)\1/) {		   # find first	doubled	char
		print "'$1' is the first doubled character\n";

	   if (/Time: (..):(..):(..)/) {   # parse out values
	       $hours =	$1;
	       $minutes	= $2;
	       $seconds	= $3;

       Several special variables also refer back to portions of	the previous
       match.  $+ returns whatever the last bracket match matched.  $& returns
       the entire matched string.  (At one point $0 did	also, but now it re-
       turns the name of the program.)	$` returns everything before the
       matched string.	$' returns everything after the	matched	string.	And
       $^N contains whatever was matched by the	most-recently closed group
       (submatch). $^N can be used in extended patterns	(see below), for exam-
       ple to assign a submatch	to a variable.

       The numbered match variables ($1, $2, $3, etc.) and the related punctu-
       ation set ($+, $&, $`, $', and $^N) are all dynamically scoped until
       the end of the enclosing	block or until the next	successful match,
       whichever comes first.  (See "Compound Statements" in perlsyn.)

       NOTE: Failed matches in Perl do not reset the match variables, which
       makes it	easier to write	code that tests	for a series of	more specific
       cases and remembers the best match.

       WARNING:	Once Perl sees that you	need one of $&,	$`, or $' anywhere in
       the program, it has to provide them for every pattern match.  This may
       substantially slow your program.	 Perl uses the same mechanism to pro-
       duce $1,	$2, etc, so you	also pay a price for each pattern that con-
       tains capturing parentheses.  (To avoid this cost while retaining the
       grouping	behaviour, use the extended regular expression "(?: ...	)" in-
       stead.)	But if you never use $&, $` or $', then	patterns without cap-
       turing parentheses will not be penalized.  So avoid $&, $', and $` if
       you can,	but if you can't (and some algorithms really appreciate	them),
       once you've used	them once, use them at will, because you've already
       paid the	price.	As of 5.005, $&	is not so costly as the	other two.

       Backslashed metacharacters in Perl are alphanumeric, such as "\b",
       "\w", "\n".  Unlike some	other regular expression languages, there are
       no backslashed symbols that aren't alphanumeric.	 So anything that
       looks like \\, \(, \), \<, \>, \{, or \}	is always interpreted as a
       literal character, not a	metacharacter.	This was once used in a	common
       idiom to	disable	or quote the special meanings of regular expression
       metacharacters in a string that you want	to use for a pattern. Simply
       quote all non-"word" characters:

	   $pattern =~ s/(\W)/\\$1/g;

       (If "use	locale"	is set,	then this depends on the current locale.)  To-
       day it is more common to	use the	quotemeta() function or	the "\Q"
       metaquoting escape sequence to disable all metacharacters' special
       meanings	like this:


       Beware that if you put literal backslashes (those not inside interpo-
       lated variables)	between	"\Q" and "\E", double-quotish backslash	inter-
       polation	may lead to confusing results.	If you need to use literal
       backslashes within "\Q...\E", consult "Gory details of parsing quoted
       constructs" in perlop.

       Extended	Patterns

       Perl also defines a consistent extension	syntax for features not	found
       in standard tools like awk and lex.  The	syntax is a pair of parenthe-
       ses with	a question mark	as the first thing within the parentheses.
       The character after the question	mark indicates the extension.

       The stability of	these extensions varies	widely.	 Some have been	part
       of the core language for	many years.  Others are	experimental and may
       change without warning or be completely removed.	 Check the documenta-
       tion on an individual feature to	verify its current status.

       A question mark was chosen for this and for the minimal-matching	con-
       struct because 1) question marks	are rare in older regular expressions,
       and 2) whenever you see one, you	should stop and	"question" exactly
       what is going on.  That's psychology...

		 A comment.  The text is ignored.  If the "/x" modifier	en-
		 ables whitespace formatting, a	simple "#" will	suffice.  Note
		 that Perl closes the comment as soon as it sees a ")",	so
		 there is no way to put	a literal ")" in the comment.

		 One or	more embedded pattern-match modifiers, to be turned on
		 (or turned off, if preceded by	"-") for the remainder of the
		 pattern or the	remainder of the enclosing pattern group (if
		 any). This is particularly useful for dynamic patterns, such
		 as those read in from a configuration file, taken from	an ar-
		 gument, or specified in a table somewhere.  Consider the case
		 where some patterns want to be	case sensitive and some	do
		 not:  The case	insensitive ones merely	need to	include	"(?i)"
		 at the	front of the pattern.  For example:

		     $pattern =	"foobar";
		     if	( /$pattern/i )	{ }

		     # more flexible:

		     $pattern =	"(?i)foobar";
		     if	( /$pattern/ ) { }

		 These modifiers are restored at the end of the	enclosing
		 group.	For example,

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

		 will match "blah" in any case,	some spaces, and an exact (in-
		 cluding the case!)  repetition	of the previous	word, assuming
		 the "/x" modifier, and	no "/i"	modifier outside this group.

		 This is for clustering, not capturing;	it groups subexpres-
		 sions like "()", but doesn't make backreferences as "()"
		 does.	So

		     @fields = split(/\b(?:a|b|c)\b/)

		 is like

		     @fields = split(/\b(a|b|c)\b/)

		 but doesn't spit out extra fields.  It's also cheaper not to
		 capture characters if you don't need to.

		 Any letters between "?" and ":" act as	flags modifiers	as
		 with "(?imsx-imsx)".  For example,


		 is equivalent to the more verbose


		 A zero-width positive look-ahead assertion.  For example,
		 "/\w+(?=\t)/" matches a word followed by a tab, without in-
		 cluding the tab in $&.

		 A zero-width negative look-ahead assertion.  For example
		 "/foo(?!bar)/"	matches	any occurrence of "foo"	that isn't
		 followed by "bar".  Note however that look-ahead and look-be-
		 hind are NOT the same thing.  You cannot use this for

		 If you	are looking for	a "bar"	that isn't preceded by a
		 "foo",	"/(?!foo)bar/" will not	do what	you want.  That's be-
		 cause the "(?!foo)" is	just saying that the next thing	cannot
		 be "foo"--and it's not, it's a	"bar", so "foobar" will	match.
		 You would have	to do something	like "/(?!foo)" for
		 that.	 We say	"like" because there's the case	of your	"bar"
		 not having three characters before it.	 You could cover that
		 this way: "/(?:(?!foo)...|^.{0,2})bar/".  Sometimes it's
		 still easier just to say:

		     if	(/bar/ && $` !~	/foo$/)

		 For look-behind see below.

		 A zero-width positive look-behind assertion.  For example,
		 "/(?<=\t)\w+/"	matches	a word that follows a tab, without in-
		 cluding the tab in $&.	 Works only for	fixed-width look-be-

		 A zero-width negative look-behind assertion.  For example
		 "/(?<!bar)foo/" matches any occurrence	of "foo" that does not
		 follow	"bar".	Works only for fixed-width look-behind.

       "(?{ code })"
		 WARNING: This extended	regular	expression feature is consid-
		 ered highly experimental, and may be changed or deleted with-
		 out notice.

		 This zero-width assertion evaluates any embedded Perl code.
		 It always succeeds, and its "code" is not interpolated.  Cur-
		 rently, the rules to determine	where the "code" ends are
		 somewhat convoluted.

		 This feature can be used together with	the special variable
		 $^N to	capture	the results of submatches in variables without
		 having	to keep	track of the number of nested parentheses. For

		   $_ =	"The brown fox jumps over the lazy dog";
		   /the	(\S+)(?{ $color	= $^N }) (\S+)(?{ $animal = $^N	})/i;
		   print "color	= $color, animal = $animal\n";

		 Inside	the "(?{...})" block, $_ refers	to the string the reg-
		 ular expression is matching against. You can also use "pos()"
		 to know what is the current position of matching within this

		 The "code" is properly	scoped in the following	sense: If the
		 assertion is backtracked (compare "Backtracking"), all
		 changes introduced after "local"ization are undone, so	that

		   $_ =	'a' x 8;
		      (?{ $cnt = 0 })			 # Initialize $cnt.
			    local $cnt = $cnt +	1;	 # Update $cnt,	backtracking-safe.
		      (?{ $res = $cnt })		 # On success copy to non-localized
							 # location.

		 will set "$res	= 4".  Note that after the match, $cnt returns
		 to the	globally introduced value, because the scopes that re-
		 strict	"local"	operators are unwound.

		 This assertion	may be used as a "(?(condition)yes-pat-
		 tern|no-pattern)" switch.  If not used	in this	way, the re-
		 sult of evaluation of "code" is put into the special variable
		 $^R.  This happens immediately, so $^R	can be used from other
		 "(?{ code })" assertions inside the same regular expression.

		 The assignment	to $^R above is	properly localized, so the old
		 value of $^R is restored if the assertion is backtracked;
		 compare "Backtracking".

		 Due to	an unfortunate implementation issue, the Perl code
		 contained in these blocks is treated as a compile time	clo-
		 sure that can have seemingly bizarre consequences when	used
		 with lexically	scoped variables inside	of subroutines or
		 loops.	 There are various workarounds for this, including
		 simply	using global variables instead.	 If you	are using this
		 construct and strange results occur then check	for the	use of
		 lexically scoped variables.

		 For reasons of	security, this construct is forbidden if the
		 regular expression involves run-time interpolation of vari-
		 ables,	unless the perilous "use re 'eval'" pragma has been
		 used (see re),	or the variables contain results of "qr//" op-
		 erator	(see "qr/STRING/imosx" in perlop).

		 This restriction is due to the	wide-spread and	remarkably
		 convenient custom of using run-time determined	strings	as
		 patterns.  For	example:

		     $re = <>;
		     chomp $re;
		     $string =~	/$re/;

		 Before	Perl knew how to execute interpolated code within a
		 pattern, this operation was completely	safe from a security
		 point of view,	although it could raise	an exception from an
		 illegal pattern.  If you turn on the "use re 'eval'", though,
		 it is no longer secure, so you	should only do so if you are
		 also using taint checking.  Better yet, use the carefully
		 constrained evaluation	within a Safe compartment.  See
		 perlsec for details about both	these mechanisms.

		 Because Perl's	regex engine is	currently not re-entrant, in-
		 terpolated code may not invoke	the regex engine either	di-
		 rectly	with "m//" or "s///"), or indirectly with functions
		 such as "split".

       "(??{ code })"
		 WARNING: This extended	regular	expression feature is consid-
		 ered highly experimental, and may be changed or deleted with-
		 out notice.  A	simplified version of the syntax may be	intro-
		 duced for commonly used idioms.

		 This is a "postponed" regular subexpression.  The "code" is
		 evaluated at run time,	at the moment this subexpression may
		 match.	 The result of evaluation is considered	as a regular
		 expression and	matched	as if it were inserted instead of this

		 The "code" is not interpolated.  As before, the rules to de-
		 termine where the "code" ends are currently somewhat convo-

		 The following pattern matches a parenthesized group:

		   $re = qr{
				 (?> [^()]+ )	 # Non-parens without backtracking
				 (??{ $re })	 # Group with matching parens

		 Because perl's	regex engine is	not currently re-entrant, de-
		 layed code may	not invoke the regex engine either directly
		 with "m//" or "s///"),	or indirectly with functions such as

		 WARNING: This extended	regular	expression feature is consid-
		 ered highly experimental, and may be changed or deleted with-
		 out notice.

		 An "independent" subexpression, one which matches the sub-
		 string	that a standalone "pattern" would match	if anchored at
		 the given position, and it matches nothing other than this
		 substring.  This construct is useful for optimizations	of
		 what would otherwise be "eternal" matches, because it will
		 not backtrack (see "Backtracking").  It may also be useful in
		 places	where the "grab	all you	can, and do not	give anything
		 back" semantic	is desirable.

		 For example: "^(?>a*)ab" will never match, since "(?>a*)"
		 (anchored at the beginning of string, as above) will match
		 all characters	"a" at the beginning of	string,	leaving	no "a"
		 for "ab" to match.  In	contrast, "a*ab" will match the	same
		 as "a+b", since the match of the subgroup "a*"	is influenced
		 by the	following group	"ab" (see "Backtracking").  In partic-
		 ular, "a*" inside "a*ab" will match fewer characters than a
		 standalone "a*", since	this makes the tail match.

		 An effect similar to "(?>pattern)" may	be achieved by writing
		 "(?=(pattern))\1".  This matches the same substring as	a
		 standalone "a+", and the following "\1" eats the matched
		 string; it therefore makes a zero-length assertion into an
		 analogue of "(?>...)".	 (The difference between these two
		 constructs is that the	second one uses	a capturing group,
		 thus shifting ordinals	of backreferences in the rest of a
		 regular expression.)

		 Consider this pattern:

		     m{	\(
			     [^()]+		 # x+
			     \(	[^()]* \)

		 That will efficiently match a nonempty	group with matching
		 parentheses two levels	deep or	less.  However,	if there is no
		 such group, it	will take virtually forever on a long string.
		 That's	because	there are so many different ways to split a
		 long string into several substrings.  This is what "(.+)+" is
		 doing,	and "(.+)+" is similar to a subpattern of the above
		 pattern.  Consider how	the pattern above detects no-match on
		 "((()aaaaaaaaaaaaaaaaaa" in several seconds, but that each
		 extra letter doubles this time.  This exponential performance
		 will make it appear that your program has hung.  However, a
		 tiny change to	this pattern

		     m{	\(
			     (?> [^()]+	)	 # change x+ above to (?> x+ )
			     \(	[^()]* \)

		 which uses "(?>...)" matches exactly when the one above does
		 (verifying this yourself would	be a productive	exercise), but
		 finishes in a fourth the time when used on a similar string
		 with 1000000 "a"s.  Be	aware, however,	that this pattern cur-
		 rently	triggers a warning message under the "use warnings"
		 pragma	or -w switch saying it "matches	null string many times
		 in regex".

		 On simple groups, such	as the pattern "(?> [^()]+ )", a com-
		 parable effect	may be achieved	by negative look-ahead,	as in
		 "[^()]+ (?! [^()] )".	This was only 4	times slower on	a
		 string	with 1000000 "a"s.

		 The "grab all you can,	and do not give	anything back" seman-
		 tic is	desirable in many situations where on the first	sight
		 a simple "()*"	looks like the correct solution.  Suppose we
		 parse text with comments being	delimited by "#" followed by
		 some optional (horizontal) whitespace.	 Contrary to its ap-
		 pearance, "#[ \t]*" is	not the	correct	subexpression to match
		 the comment delimiter,	because	it may "give up" some white-
		 space if the remainder	of the pattern can be made to match
		 that way.  The	correct	answer is either one of	these:

		     (?>#[ \t]*)
		     #[	\t]*(?![ \t])

		 For example, to grab non-empty	comments into $1, one should
		 use either one	of these:

		     / (?> \# [	\t]* ) (	.+ ) /x;
		     /	   \# [	\t]*   ( [^ \t]	.* ) /x;

		 Which one you pick depends on which of	these expressions bet-
		 ter reflects the above	specification of comments.

		 WARNING: This extended	regular	expression feature is consid-
		 ered highly experimental, and may be changed or deleted with-
		 out notice.

		 Conditional expression.  "(condition)"	should be either an
		 integer in parentheses	(which is valid	if the corresponding
		 pair of parentheses matched), or look-ahead/look-behind/eval-
		 uate zero-width assertion.

		 For example:

		     m{	( \( )?
			(?(1) \) )

		 matches a chunk of non-parentheses, possibly included in
		 parentheses themselves.


       NOTE: This section presents an abstract approximation of	regular	ex-
       pression	behavior.  For a more rigorous (and complicated) view of the
       rules involved in selecting a match among possible alternatives,	see
       "Combining RE Pieces".

       A fundamental feature of	regular	expression matching involves the no-
       tion called backtracking, which is currently used (when needed) by all
       regular non-possessive expression quantifiers, namely "*", "*?",	"+",
       "+?", "{n,m}", and "{n,m}?".  Backtracking is often optimized inter-
       nally, but the general principle	outlined here is valid.

       For a regular expression	to match, the entire regular expression	must
       match, not just part of it.  So if the beginning	of a pattern contain-
       ing a quantifier	succeeds in a way that causes later parts in the pat-
       tern to fail, the matching engine backs up and recalculates the begin-
       ning part--that's why it's called backtracking.

       Here is an example of backtracking:  Let's say you want to find the
       word following "foo" in the string "Food	is on the foo table.":

	   $_ =	"Food is on the	foo table.";
	   if (	/\b(foo)\s+(\w+)/i ) {
	       print "$2 follows $1.\n";

       When the	match runs, the	first part of the regular expression
       ("\b(foo)") finds a possible match right	at the beginning of the
       string, and loads up $1 with "Foo".  However, as	soon as	the matching
       engine sees that	there's	no whitespace following	the "Foo" that it had
       saved in	$1, it realizes	its mistake and	starts over again one charac-
       ter after where it had the tentative match.  This time it goes all the
       way until the next occurrence of	"foo". The complete regular expression
       matches this time, and you get the expected output of "table follows

       Sometimes minimal matching can help a lot.  Imagine you'd like to match
       everything between "foo"	and "bar".  Initially, you write something
       like this:

	   $_ =	 "The food is under the	bar in the barn.";
	   if (	/foo(.*)bar/ ) {
	       print "got <$1>\n";

       Which perhaps unexpectedly yields:

	 got <d	is under the bar in the	>

       That's because ".*" was greedy, so you get everything between the first
       "foo" and the last "bar".  Here it's more effective to use minimal
       matching	to make	sure you get the text between a	"foo" and the first
       "bar" thereafter.

	   if (	/foo(.*?)bar/ )	{ print	"got <$1>\n" }
	 got <d	is under the >

       Here's another example. Let's say you'd like to match a number at the
       end of a	string,	and you	also want to keep the preceding	part of	the
       match.  So you write this:

	   $_ =	"I have	2 numbers: 53147";
	   if (	/(.*)(\d*)/ ) {				       # Wrong!
	       print "Beginning	is <$1>, number	is <$2>.\n";

       That won't work at all, because ".*" was	greedy and gobbled up the
       whole string. As	"\d*" can match	on an empty string the complete	regu-
       lar expression matched successfully.

	   Beginning is	<I have	2 numbers: 53147>, number is <>.

       Here are	some variants, most of which don't work:

	   $_ =	"I have	2 numbers: 53147";
	   @pats = qw{

	   for $pat (@pats) {
	       printf "%-12s ",	$pat;
	       if ( /$pat/ ) {
		   print "<$1> <$2>\n";
	       } else {
		   print "FAIL\n";

       That will print out:

	   (.*)(\d*)	<I have	2 numbers: 53147> <>
	   (.*)(\d+)	<I have	2 numbers: 5314> <7>
	   (.*?)(\d*)	<> <>
	   (.*?)(\d+)	<I have	> <2>
	   (.*)(\d+)$	<I have	2 numbers: 5314> <7>
	   (.*?)(\d+)$	<I have	2 numbers: > <53147>
	   (.*)\b(\d+)$	<I have	2 numbers: > <53147>
	   (.*\D)(\d+)$	<I have	2 numbers: > <53147>

       As you see, this	can be a bit tricky.  It's important to	realize	that a
       regular expression is merely a set of assertions	that gives a defini-
       tion of success.	 There may be 0, 1, or several different ways that the
       definition might	succeed	against	a particular string.  And if there are
       multiple	ways it	might succeed, you need	to understand backtracking to
       know which variety of success you will achieve.

       When using look-ahead assertions	and negations, this can	all get	even
       trickier.  Imagine you'd	like to	find a sequence	of non-digits not fol-
       lowed by	"123".	You might try to write that as

	   $_ =	"ABC123";
	   if (	/^\D*(?!123)/ )	{	       # Wrong!
	       print "Yup, no 123 in $_\n";

       But that	isn't going to match; at least,	not the	way you're hoping.  It
       claims that there is no 123 in the string.  Here's a clearer picture of
       why that	pattern	matches, contrary to popular expectations:

	   $x =	'ABC123';
	   $y =	'ABC445';

	   print "1: got $1\n" if $x =~	/^(ABC)(?!123)/;
	   print "2: got $1\n" if $y =~	/^(ABC)(?!123)/;

	   print "3: got $1\n" if $x =~	/^(\D*)(?!123)/;
	   print "4: got $1\n" if $y =~	/^(\D*)(?!123)/;

       This prints

	   2: got ABC
	   3: got AB
	   4: got ABC

       You might have expected test 3 to fail because it seems to a more gen-
       eral purpose version of test 1.	The important difference between them
       is that test 3 contains a quantifier ("\D*") and	so can use backtrack-
       ing, whereas test 1 will	not.  What's happening is that you've asked
       "Is it true that	at the start of	$x, following 0	or more	non-digits,
       you have	something that's not 123?"  If the pattern matcher had let
       "\D*" expand to "ABC", this would have caused the whole pattern to

       The search engine will initially	match "\D*" with "ABC".	 Then it will
       try to match "(?!123" with "123", which fails.  But because a quanti-
       fier ("\D*") has	been used in the regular expression, the search	engine
       can backtrack and retry the match differently in	the hope of matching
       the complete regular expression.

       The pattern really, really wants	to succeed, so it uses the standard
       pattern back-off-and-retry and lets "\D*" expand	to just	"AB" this
       time.  Now there's indeed something following "AB" that is not "123".
       It's "C123", which suffices.

       We can deal with	this by	using both an assertion	and a negation.	 We'll
       say that	the first part in $1 must be followed both by a	digit and by
       something that's	not "123".  Remember that the look-aheads are zero-
       width expressions--they only look, but don't consume any	of the string
       in their	match.	So rewriting this way produces what you'd expect; that
       is, case	5 will fail, but case 6	succeeds:

	   print "5: got $1\n" if $x =~	/^(\D*)(?=\d)(?!123)/;
	   print "6: got $1\n" if $y =~	/^(\D*)(?=\d)(?!123)/;

	   6: got ABC

       In other	words, the two zero-width assertions next to each other	work
       as though they're ANDed together, just as you'd use any built-in	asser-
       tions:  "/^$/" matches only if you're at	the beginning of the line AND
       the end of the line simultaneously.  The	deeper underlying truth	is
       that juxtaposition in regular expressions always	means AND, except when
       you write an explicit OR	using the vertical bar.	 "/ab/"	means match
       "a" AND (then) match "b", although the attempted	matches	are made at
       different positions because "a" is not a	zero-width assertion, but a
       one-width assertion.

       WARNING:	Particularly complicated regular expressions can take exponen-
       tial time to solve because of the immense number	of possible ways they
       can use backtracking to try for a match.	 For example, without internal
       optimizations done by the regular expression engine, this will take a
       painfully long time to run:

	   'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/

       And if you used "*"'s in	the internal groups instead of limiting	them
       to 0 through 5 matches, then it would take forever--or until you	ran
       out of stack space.  Moreover, these internal optimizations are not al-
       ways applicable.	 For example, if you put "{0,5}" instead of "*"	on the
       external	group, no current optimization is applicable, and the match
       takes a long time to finish.

       A powerful tool for optimizing such beasts is what is known as an "in-
       dependent group", which does not	backtrack (see ""(?>pattern)"").  Note
       also that zero-length look-ahead/look-behind assertions will not	back-
       track to	make the tail match, since they	are in "logical" context: only
       whether they match is considered	relevant.  For an example where	side-
       effects of look-ahead might have	influenced the following match,	see

       Version 8 Regular Expressions

       In case you're not familiar with	the "regular" Version 8	regex rou-
       tines, here are the pattern-matching rules not described	above.

       Any single character matches itself, unless it is a metacharacter with
       a special meaning described here	or above.  You can cause characters
       that normally function as metacharacters	to be interpreted literally by
       prefixing them with a "\" (e.g.,	"\." matches a ".", not	any character;
       "\\" matches a "\"). This escape	mechanism is also required for the
       character used as the pattern delimiter.

       A series	of characters matches that series of characters	in the target
       string, so the pattern  "blurfl"	would match "blurfl" in	the target

       You can specify a character class, by enclosing a list of characters in
       "[]", which will	match any one character	from the list.	If the first
       character after the "[" is "^", the class matches any character not in
       the list.  Within a list, the "-" character specifies a range, so that
       "a-z" represents	all characters between "a" and "z", inclusive.	If you
       want either "-" or "]" itself to	be a member of a class,	put it at the
       start of	the list (possibly after a "^"), or escape it with a back-
       slash.  "-" is also taken literally when	it is at the end of the	list,
       just before the closing "]".  (The following all	specify	the same class
       of three	characters: "[-az]", "[az-]", and "[a\-z]".  All are different
       from "[a-z]", which specifies a class containing	twenty-six characters,
       even on EBCDIC based coded character sets.)  Also, if you try to	use
       the character classes "\w", "\W", "\s", "\S", "\d", or "\D" as end-
       points of a range, that's not a range, the "-" is understood literally.

       Note also that the whole	range idea is rather unportable	between	char-
       acter sets--and even within character sets they may cause results you
       probably	didn't expect.	A sound	principle is to	use only ranges	that
       begin from and end at either alphabetics	of equal case ([a-e], [A-E]),
       or digits ([0-9]).  Anything else is unsafe.  If	in doubt, spell	out
       the character sets in full.

       Characters may be specified using a metacharacter syntax	much like that
       used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
       "\f" a form feed, etc.  More generally, \nnn, where nnn is a string of
       octal digits, matches the character whose coded character set value is
       nnn.  Similarly,	\xnn, where nn are hexadecimal digits, matches the
       character whose numeric value is	nn. The	expression \cx matches the
       character control-x.  Finally, the "." metacharacter matches any	char-
       acter except "\n" (unless you use "/s").

       You can specify a series	of alternatives	for a pattern using "|"	to
       separate	them, so that "fee|fie|foe" will match any of "fee", "fie", or
       "foe" in	the target string (as would "f(e|i|o)e").  The first alterna-
       tive includes everything	from the last pattern delimiter	("(", "[", or
       the beginning of	the pattern) up	to the first "|", and the last alter-
       native contains everything from the last	"|" to the next	pattern	delim-
       iter.  That's why it's common practice to include alternatives in
       parentheses: to minimize	confusion about	where they start and end.

       Alternatives are	tried from left	to right, so the first alternative
       found for which the entire expression matches, is the one that is cho-
       sen. This means that alternatives are not necessarily greedy. For exam-
       ple: when matching "foo|foot" against "barefoot", only the "foo"	part
       will match, as that is the first	alternative tried, and it successfully
       matches the target string. (This	might not seem important, but it is
       important when you are capturing	matched	text using parentheses.)

       Also remember that "|" is interpreted as	a literal within square	brack-
       ets, so if you write "[fee|fie|foe]" you're really only matching

       Within a	pattern, you may designate subpatterns for later reference by
       enclosing them in parentheses, and you may refer	back to	the nth	sub-
       pattern later in	the pattern using the metacharacter \n.	 Subpatterns
       are numbered based on the left to right order of	their opening paren-
       thesis.	A backreference	matches	whatever actually matched the subpat-
       tern in the string being	examined, not the rules	for that subpattern.
       Therefore, "(0|0x)\d*\s\1\d*" will match	"0x1234	0x4321", but not
       "0x1234 01234", because subpattern 1 matched "0x", even though the rule
       "0|0x" could potentially	match the leading 0 in the second number.

       Warning on \1 Instead of	$1

       Some people get too used	to writing things like:

	   $pattern =~ s/(\W)/\\\1/g;

       This is grandfathered for the RHS of a substitute to avoid shocking the
       sed addicts, but	it's a dirty habit to get into.	 That's	because	in
       PerlThink, the righthand	side of	an "s///" is a double-quoted string.
       "\1" in the usual double-quoted string means a control-A.  The custom-
       ary Unix	meaning	of "\1"	is kludged in for "s///".  However, if you get
       into the	habit of doing that, you get yourself into trouble if you then
       add an "/e" modifier.

	   s/(\d+)/ \1 + 1 /eg;	       # causes	warning	under -w

       Or if you try to	do


       You can't disambiguate that by saying "\{1}000",	whereas	you can	fix it
       with "${1}000".	The operation of interpolation should not be confused
       with the	operation of matching a	backreference.	Certainly they mean
       two different things on the left	side of	the "s///".

       Repeated	Patterns Matching a Zero-length	Substring

       WARNING:	Difficult material (and	prose) ahead.  This section needs a

       Regular expressions provide a terse and powerful	programming language.
       As with most other power	tools, power comes together with the ability
       to wreak	havoc.

       A common	abuse of this power stems from the ability to make infinite
       loops using regular expressions,	with something as innocuous as:

	   'foo' =~ m{ ( o? )* }x;

       The "o?"	matches	at the beginning of 'foo', and since the position in
       the string is not moved by the match, "o?" would	match again and	again
       because of the "*" quantifier.  Another common way to create a similar
       cycle is	with the looping modifier "//g":

	   @matches = (	'foo' =~ m{ o? }xg );


	   print "match: <$&>\n" while 'foo' =~	m{ o? }xg;

       or the loop implied by split().

       However,	long experience	has shown that many programming	tasks may be
       significantly simplified	by using repeated subexpressions that may
       match zero-length substrings.  Here's a simple example being:

	   @chars = split //, $string;		 # // is not magic in split
	   ($whitewashed = $string) =~ s/()/ /g; # parens avoid	magic s// /

       Thus Perl allows	such constructs, by forcefully breaking	the infinite
       loop.  The rules	for this are different for lower-level loops given by
       the greedy quantifiers "*+{}", and for higher-level ones	like the "/g"
       modifier	or split() operator.

       The lower-level loops are interrupted (that is, the loop	is broken)
       when Perl detects that a	repeated expression matched a zero-length sub-
       string.	 Thus

	  m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;

       is made equivalent to

	  m{   (?: NON_ZERO_LENGTH )*
	       (?: ZERO_LENGTH )?

       The higher level-loops preserve an additional state between iterations:
       whether the last	match was zero-length.	To break the loop, the follow-
       ing match after a zero-length match is prohibited to have a length of
       zero.  This prohibition interacts with backtracking (see	"Backtrack-
       ing"), and so the second	best match is chosen if	the best match is of
       zero length.

       For example:

	   $_ =	'bar';

       results in "<><b><><a><><r><>".	At each	position of the	string the
       best match given	by non-greedy "??" is the zero-length match, and the
       second best match is what is matched by "\w".  Thus zero-length matches
       alternate with one-character-long matches.

       Similarly, for repeated "m/()/g"	the second-best	match is the match at
       the position one	notch further in the string.

       The additional state of being matched with zero-length is associated
       with the	matched	string,	and is reset by	each assignment	to pos().
       Zero-length matches at the end of the previous match are	ignored	during

       Combining RE Pieces

       Each of the elementary pieces of	regular	expressions which were de-
       scribed before (such as "ab" or "\Z") could match at most one substring
       at the given position of	the input string.  However, in a typical regu-
       lar expression these elementary pieces are combined into	more compli-
       cated patterns using combining operators	"ST", "S|T", "S*" etc (in
       these examples "S" and "T" are regular subexpressions).

       Such combinations can include alternatives, leading to a	problem	of
       choice: if we match a regular expression	"a|ab" against "abc", will it
       match substring "a" or "ab"?  One way to	describe which substring is
       actually	matched	is the concept of backtracking (see "Backtracking").
       However,	this description is too	low-level and makes you	think in terms
       of a particular implementation.

       Another description starts with notions of "better"/"worse".  All the
       substrings which	may be matched by the given regular expression can be
       sorted from the "best" match to the "worst" match, and it is the	"best"
       match which is chosen.  This substitutes	the question of	"what is cho-
       sen?"  by the question of "which	matches	are better, and	which are

       Again, for elementary pieces there is no	such question, since at	most
       one match at a given position is	possible.  This	section	describes the
       notion of better/worse for combining operators.	In the description be-
       low "S" and "T" are regular subexpressions.

	   Consider two	possible matches, "AB" and "A'B'", "A" and "A'"	are
	   substrings which can	be matched by "S", "B" and "B'"	are substrings
	   which can be	matched	by "T".

	   If "A" is better match for "S" than "A'", "AB" is a better match
	   than	"A'B'".

	   If "A" and "A'" coincide: "AB" is a better match than "AB'" if "B"
	   is better match for "T" than	"B'".

	   When	"S" can	match, it is a better match than when only "T" can

	   Ordering of two matches for "S" is the same as for "S".  Similar
	   for two matches for "T".

	   Matches as "SSS...S"	(repeated as many times	as necessary).

	   Matches as "S{max}|S{max-1}|...|S{min+1}|S{min}".

	   Matches as "S{min}|S{min+1}|...|S{max-1}|S{max}".

       "S?", "S*", "S+"
	   Same	as "S{0,1}", "S{0,BIG_NUMBER}",	"S{1,BIG_NUMBER}" respec-

       "S??", "S*?", "S+?"
	   Same	as "S{0,1}?", "S{0,BIG_NUMBER}?", "S{1,BIG_NUMBER}?" respec-

	   Matches the best match for "S" and only that.

       "(?=S)",	"(?<=S)"
	   Only	the best match for "S" is considered.  (This is	important only
	   if "S" has capturing	parentheses, and backreferences	are used some-
	   where else in the whole regular expression.)

       "(?!S)",	"(?<!S)"
	   For this grouping operator there is no need to describe the order-
	   ing,	since only whether or not "S" can match	is important.

       "(??{ EXPR })"
	   The ordering	is the same as for the regular expression which	is the
	   result of EXPR.

	   Recall that which of	"yes-pattern" or "no-pattern" actually matches
	   is already determined.  The ordering	of the matches is the same as
	   for the chosen subexpression.

       The above recipes describe the ordering of matches at a given position.
       One more	rule is	needed to understand how a match is determined for the
       whole regular expression: a match at an earlier position	is always bet-
       ter than	a match	at a later position.

       Creating	Custom RE Engines

       Overloaded constants (see overload) provide a simple way	to extend the
       functionality of	the RE engine.

       Suppose that we want to enable a	new RE escape-sequence "\Y|" which
       matches at a boundary between whitespace	characters and non-whitespace
       characters.  Note that "(?=\S)(?<!\S)|(?!\S)(?<=\S)" matches exactly at
       these positions,	so we want to have each	"\Y|" in the place of the more
       complicated version.  We	can create a module "customre" to do this:

	   package customre;
	   use overload;

	   sub import {
	     die "No argument to customre::import allowed" if @_;
	     overload::constant	'qr' =>	\&convert;

	   sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}

	   # We	must also take care of not escaping the	legitimate \\Y|
	   # sequence, hence the presence of '\\' in the conversion rules.
	   my %rules = ( '\\' => '\\\\',
			 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
	   sub convert {
	     my	$re = shift;
	     $re =~ s{
		       \\ ( \\ | Y . )
		     { $rules{$1} or invalid($re,$1) }sgex;
	     return $re;

       Now "use	customre" enables the new escape in constant regular expres-
       sions, i.e., those without any runtime variable interpolations.	As
       documented in overload, this conversion will work only over literal
       parts of	regular	expressions.  For "\Y|$re\Y|" the variable part	of
       this regular expression needs to	be converted explicitly	(but only if
       the special meaning of "\Y|" should be enabled inside $re):

	   use customre;
	   $re = <>;
	   chomp $re;
	   $re = customre::convert $re;

       This document varies from difficult to understand to completely and ut-
       terly opaque.  The wandering prose riddled with jargon is hard to
       fathom in several places.

       This document needs a rewrite that separates the	tutorial content from
       the reference content.



       "Regexp Quote-Like Operators" in	perlop.

       "Gory details of	parsing	quoted constructs" in perlop.


       "pos" in	perlfunc.



       Mastering Regular Expressions by	Jeffrey	Friedl,	published by O'Reilly
       and Associates.

perl v5.8.9			  2007-11-17			     PERLRE(1)


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