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REGEX(3)	       FreeBSD Library Functions Manual		      REGEX(3)

NAME
     regex, regcomp, regexec, regerror,	regfree, regasub, regnsub -- regular-
     expression	library

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <regex.h>

     int
     regcomp(regex_t * restrict	preg, const char * restrict pattern,
	 int cflags);

     int
     regexec(const regex_t * restrict preg, const char * restrict string,
	 size_t	nmatch,	regmatch_t pmatch[], int eflags);

     size_t
     regerror(int errcode, const regex_t * restrict preg,
	 char *	restrict errbuf, size_t	errbuf_size);

     void
     regfree(regex_t *preg);

     ssize_t
     regnsub(char *buf,	size_t bufsiz, const char *sub,	const regmatch_t *rm,
	 const char *str);

     ssize_t
     regasub(char **buf, const char *sub, const	regmatch_t *rm,
	 const char *sstr);

DESCRIPTION
     These routines implement IEEE Std 1003.2-1992 ("POSIX.2") regular expres-
     sions (``RE''s); see re_format(7).	 regcomp() compiles an RE written as a
     string into an internal form, regexec() matches that internal form
     against a string and reports results, regerror() transforms error codes
     from either into human-readable messages, and regfree() frees any dynami-
     cally-allocated storage used by the internal form of an RE.

     The header	<regex.h> declares two structure types,	regex_t	and
     regmatch_t, the former for	compiled internal forms	and the	latter for
     match reporting.  It also declares	the four functions, a type regoff_t,
     and a number of constants with names starting with	``REG_''.

     regcomp() compiles	the regular expression contained in the	pattern
     string, subject to	the flags in cflags, and places	the results in the
     regex_t structure pointed to by preg.  cflags is the bitwise OR of	zero
     or	more of	the following flags:

     REG_EXTENDED     Compile modern (``extended'') REs, rather	than the obso-
		      lete (``basic'') REs that	are the	default.

     REG_BASIC	      This is a	synonym	for 0, provided	as a counterpart to
		      REG_EXTENDED to improve readability.

     REG_NOSPEC	      Compile with recognition of all special characters
		      turned off.  All characters are thus considered ordi-
		      nary, so the ``RE'' is a literal string.	This is	an ex-
		      tension, compatible with but not specified by IEEE Std
		      1003.2-1992 ("POSIX.2"), and should be used with caution
		      in software intended to be portable to other systems.
		      REG_EXTENDED and REG_NOSPEC may not be used in the same
		      call to regcomp().

     REG_ICASE	      Compile for matching that	ignores	upper/lower case dis-
		      tinctions.  See re_format(7).

     REG_NOSUB	      Compile for matching that	need only report success or
		      failure, not what	was matched.

     REG_NEWLINE      Compile for newline-sensitive matching.  By default,
		      newline is a completely ordinary character with no spe-
		      cial meaning in either REs or strings.  With this	flag,
		      `[^' bracket expressions and `.' never match newline, a
		      `^' anchor matches the null string after any newline in
		      the string in addition to	its normal function, and the
		      `$' anchor matches the null string before	any newline in
		      the string in addition to	its normal function.

     REG_PEND	      The regular expression ends, not at the first NUL, but
		      just before the character	pointed	to by the re_endp mem-
		      ber of the structure pointed to by preg.	The re_endp
		      member is	of type	const char *.  This flag permits in-
		      clusion of NULs in the RE; they are considered ordinary
		      characters.  This	is an extension, compatible with but
		      not specified by IEEE Std	1003.2-1992 ("POSIX.2"), and
		      should be	used with caution in software intended to be
		      portable to other	systems.

     When successful, regcomp()	returns	0 and fills in the structure pointed
     to	by preg.  One member of	that structure (other than re_endp) is publi-
     cized: re_nsub, of	type size_t, contains the number of parenthesized sub-
     expressions within	the RE (except that the	value of this member is	unde-
     fined if the REG_NOSUB flag was used).  If	regcomp() fails, it returns a
     non-zero error code; see DIAGNOSTICS.

     regexec() matches the compiled RE pointed to by preg against the string,
     subject to	the flags in eflags, and reports results using nmatch, pmatch,
     and the returned value.  The RE must have been compiled by	a previous in-
     vocation of regcomp().  The compiled form is not altered during execution
     of	regexec(), so a	single compiled	RE can be used simultaneously by mul-
     tiple threads.

     By	default, the NUL-terminated string pointed to by string	is considered
     to	be the text of an entire line, minus any terminating newline.  The
     eflags argument is	the bitwise OR of zero or more of the following	flags:

     REG_NOTBOL	    The	first character	of the string is not the beginning of
		    a line, so the `^' anchor should not match before it.
		    This does not affect the behavior of newlines under
		    REG_NEWLINE.

     REG_NOTEOL	    The	NUL terminating	the string does	not end	a line,	so the
		    `$'	anchor should not match	before it.  This does not af-
		    fect the behavior of newlines under	REG_NEWLINE.

     REG_STARTEND   The	string is considered to	start at string	+
		    pmatch[0].rm_so and	to have	a terminating NUL located at
		    string + pmatch[0].rm_eo (there need not actually be a NUL
		    at that location), regardless of the value of nmatch.  See
		    below for the definition of	pmatch and nmatch.  This is an
		    extension, compatible with but not specified by IEEE Std
		    1003.2-1992	("POSIX.2"), and should	be used	with caution
		    in software	intended to be portable	to other systems.
		    Note that a	non-zero rm_so does not	imply REG_NOTBOL;
		    REG_STARTEND affects only the location of the string, not
		    how	it is matched.

     See re_format(7) for a discussion of what is matched in situations	where
     an	RE or a	portion	thereof	could match any	of several substrings of
     string.

     Normally, regexec() returns 0 for success and the non-zero	code
     REG_NOMATCH for failure.  Other non-zero error codes may be returned in
     exceptional situations; see DIAGNOSTICS.

     If	REG_NOSUB was specified	in the compilation of the RE, or if nmatch is
     0,	regexec() ignores the pmatch argument (but see below for the case
     where REG_STARTEND	is specified).	Otherwise, pmatch points to an array
     of	nmatch structures of type regmatch_t.  Such a structure	has at least
     the members rm_so and rm_eo, both of type regoff_t	(a signed arithmetic
     type at least as large as an off_t	and a ssize_t),	containing respec-
     tively the	offset of the first character of a substring and the offset of
     the first character after the end of the substring.  Offsets are measured
     from the beginning	of the string argument given to	regexec().  An empty
     substring is denoted by equal offsets, both indicating the	character fol-
     lowing the	empty substring.

     The 0th member of the pmatch array	is filled in to	indicate what sub-
     string of string was matched by the entire	RE.  Remaining members report
     what substring was	matched	by parenthesized subexpressions	within the RE;
     member i reports subexpression i, with subexpressions counted (starting
     at	1) by the order	of their opening parentheses in	the RE,	left to	right.
     Unused entries in the array--corresponding	either to subexpressions that
     did not participate in the	match at all, or to subexpressions that	do not
     exist in the RE (that is, i > preg-_re_nsub) --have both rm_so and	rm_eo
     set to -1.	 If a subexpression participated in the	match several times,
     the reported substring is the last	one it matched.	 (Note,	as an example
     in	particular, that when the RE `(b*)+' matches `bbb', the	parenthesized
     subexpression matches each	of the three `b's and then an infinite number
     of	empty strings following	the last `b', so the reported substring	is one
     of	the empties.)

     If	REG_STARTEND is	specified, pmatch must point to	at least one
     regmatch_t	(even if nmatch	is 0 or	REG_NOSUB was specified), to hold the
     input offsets for REG_STARTEND.  Use for output is	still entirely con-
     trolled by	nmatch;	if nmatch is 0 or REG_NOSUB was	specified, the value
     of	pmatch [0] will	not be changed by a successful regexec().

     regerror()	maps a non-zero	errcode	from either regcomp() or regexec() to
     a human-readable, printable message.  If preg is non-NULL,	the error code
     should have arisen	from use of the	regex_t	pointed	to by preg, and	if the
     error code	came from regcomp(), it	should have been the result from the
     most recent regcomp() using that regex_t.	(regerror() may	be able	to
     supply a more detailed message using information from the regex_t.)
     regerror()	places the NUL-terminated message into the buffer pointed to
     by	errbuf,	limiting the length (including the NUL)	to at most errbuf_size
     bytes.  If	the whole message won't	fit, as	much of	it as will fit before
     the terminating NUL is supplied.  In any case, the	returned value is the
     size of buffer needed to hold the whole message (including	terminating
     NUL).  If errbuf_size is 0, errbuf	is ignored but the return value	is
     still correct.

     If	the errcode given to regerror()	is first ORed with REG_ITOA, the
     ``message'' that results is the printable name of the error code, e.g.
     ``REG_NOMATCH'', rather than an explanation thereof.  If errcode is
     REG_ATOI, then preg shall be non-NULL and the re_endp member of the
     structure it points to must point to the printable	name of	an error code;
     in	this case, the result in errbuf	is the decimal digits of the numeric
     value of the error	code (0	if the name is not recognized).	 REG_ITOA and
     REG_ATOI are intended primarily as	debugging facilities; they are exten-
     sions, compatible with but	not specified by IEEE Std 1003.2-1992
     ("POSIX.2"), and should be	used with caution in software intended to be
     portable to other systems.	 Be warned also	that they are considered ex-
     perimental	and changes are	possible.

     regfree() frees any dynamically-allocated storage associated with the
     compiled RE pointed to by preg.  The remaining regex_t is no longer a
     valid compiled RE and the effect of supplying it to regexec() or
     regerror()	is undefined.

     None of these functions references	global variables except	for tables of
     constants;	all are	safe for use from multiple threads if the arguments
     are safe.

     The regnsub() and regasub() functions perform substitutions using sed(1)
     like syntax.  They	return the length of the string	that would have	been
     created if	there was enough space or -1 on	error, setting errno.  The re-
     sult is being placed in buf which is user-supplied	in regnsub() and dy-
     namically allocated in regasub().	The sub	argument contains a substitu-
     tion string which might refer to the first	9 regular expression strings
     using "\<n>" to refer to the nth matched item, or "&" (which is equiva-
     lent to "\0") to refer to the full	match.	The rm array must be at	least
     10	elements long, and should contain the result of	the matches from a
     previous regexec()	call.  Only 10 elements	of the rm array	can be used.
     The str argument contains the source string to apply the transformation
     to.

IMPLEMENTATION CHOICES
     There are a number	of decisions that IEEE Std 1003.2-1992 ("POSIX.2")
     leaves up to the implementor, either by explicitly	saying ``undefined''
     or	by virtue of them being	forbidden by the RE grammar.  This implementa-
     tion treats them as follows.

     See re_format(7) for a discussion of the definition of case-independent
     matching.

     There is no particular limit on the length	of REs,	except insofar as mem-
     ory is limited.  Memory usage is approximately linear in RE size, and
     largely insensitive to RE complexity, except for bounded repetitions.
     See BUGS for one short RE using them that will run	almost any system out
     of	memory.

     A backslashed character other than	one specifically given a magic meaning
     by	IEEE Std 1003.2-1992 ("POSIX.2") (such magic meanings occur only in
     obsolete [``basic''] REs) is taken	as an ordinary character.

     Any unmatched [ is	a REG_EBRACK error.

     Equivalence classes cannot	begin or end bracket-expression	ranges.	 The
     endpoint of one range cannot begin	another.

     RE_DUP_MAX, the limit on repetition counts	in bounded repetitions,	is
     255.

     A repetition operator (?, *, +, or	bounds)	cannot follow another repeti-
     tion operator.  A repetition operator cannot begin	an expression or sub-
     expression	or follow `^' or `|'.

     `|' cannot	appear first or	last in	a (sub)expression or after another
     `|', i.e. an operand of `|' cannot	be an empty subexpression.  An empty
     parenthesized subexpression, `()',	is legal and matches an	empty
     (sub)string.  An empty string is not a legal RE.

     A `{' followed by a digit is considered the beginning of bounds for a
     bounded repetition, which must then follow	the syntax for bounds.	A `{'
     not followed by a digit is	considered an ordinary character.

     `^' and `$' beginning and ending subexpressions in	obsolete (``basic'')
     REs are anchors, not ordinary characters.

DIAGNOSTICS
     Non-zero error codes from regcomp() and regexec() include the following:

     REG_NOMATCH      regexec()	failed to match
     REG_BADPAT	      invalid regular expression
     REG_ECOLLATE     invalid collating	element
     REG_ECTYPE	      invalid character	class
     REG_EESCAPE      \	applied	to unescapable character
     REG_ESUBREG      invalid backreference number
     REG_EBRACK	      brackets [ ] not balanced
     REG_EPAREN	      parentheses ( ) not balanced
     REG_EBRACE	      braces { } not balanced
     REG_BADBR	      invalid repetition count(s) in { }
     REG_ERANGE	      invalid character	range in [ ]
     REG_ESPACE	      ran out of memory
     REG_BADRPT	      ?, *, or + operand invalid
     REG_EMPTY	      empty (sub)expression
     REG_ASSERT	      ``can't happen''--you found a bug
     REG_INVARG	      invalid argument,	e.g. negative-length string

SEE ALSO
     grep(1), sed(1), re_format(7)

     IEEE Std 1003.2-1992 ("POSIX.2"), sections	2.8 (Regular Expression	Nota-
     tion) and B.5 (C Binding for Regular Expression Matching).

HISTORY
     Originally	written	by Henry Spencer.  Altered for inclusion in the	4.4BSD
     distribution.

     The regnsub() and regasub() functions appeared in NetBSD 8.

BUGS
     There is one known	functionality bug.  The	implementation of internation-
     alization is incomplete: the locale is always assumed to be the default
     one of IEEE Std 1003.2-1992 ("POSIX.2"), and only the collating elements
     etc. of that locale are available.

     The back-reference	code is	subtle and doubts linger about its correctness
     in	complex	cases.

     regexec() performance is poor.  This will improve with later releases.
     nmatch exceeding 0	is expensive; nmatch exceeding 1 is worse.  regexec is
     largely insensitive to RE complexity except that back references are mas-
     sively expensive.	RE length does matter; in particular, there is a
     strong speed bonus	for keeping RE length under about 30 characters, with
     most special characters counting roughly double.

     regcomp() implements bounded repetitions by macro expansion, which	is
     costly in time and	space if counts	are large or bounded repetitions are
     nested.  An RE like, say, `((((a{1,100}){1,100}){1,100}){1,100}){1,100}'
     will (eventually) run almost any existing machine out of swap space.

     There are suspected problems with response	to obscure error conditions.
     Notably, certain kinds of internal	overflow, produced only	by truly enor-
     mous REs or by multiply nested bounded repetitions, are probably not han-
     dled well.

     Due to a mistake in IEEE Std 1003.2-1992 ("POSIX.2"), things like `a)b'
     are legal REs because `)' is a special character only in the presence of
     a previous	unmatched `('.	This can't be fixed until the spec is fixed.

     The standard's definition of back references is vague.  For example, does
     `a\(\(b\)*\2\)*d' match `abbbd'?  Until the standard is clarified,	behav-
     ior in such cases should not be relied on.

     The implementation	of word-boundary matching is a bit of a	kludge,	and
     bugs may lurk in combinations of word-boundary matching and anchoring.

FreeBSD	13.0		       February	26, 2018		  FreeBSD 13.0

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | IMPLEMENTATION CHOICES | DIAGNOSTICS | SEE ALSO | HISTORY | BUGS

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