Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
REGEX(3)	       FreeBSD Library Functions Manual		      REGEX(3)

     regcomp, regexec, regerror, regfree -- regular-expression library

     Standard C	Library	(libc, -lc)

     #include <sys/types.h>
     #include <regex.h>

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

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

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

     regfree(regex_t *preg);

     These routines implement IEEE Std 1003.2 (``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 obsolete
		   (``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 ordinary, so the
		   ``RE'' is a literal string.	This is	an extension, compati-
		   ble with but	not specified by IEEE Std 1003.2
		   (``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 distinc-
		   tions.  See re_format(7).

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

     REG_NEWLINE   Compile for newline-sensitive matching.  By default,	new-
		   line	is a completely	ordinary character with	no special
		   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 member of
		   the structure pointed to by preg.  The re_endp member is of
		   type	const char *.  This flag permits inclusion of NULs in
		   the RE; they	are considered ordinary	characters.  This is
		   an extension, compatible with but not specified by IEEE Std
		   1003.2 (``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
     subexpressions within the RE (except that the value of this member	is
     undefined 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
     invocation	of regcomp().  The compiled form is not	altered	during execu-
     tion of regexec(),	so a single compiled RE	can be used simultaneously by
     multiple 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
		   affect 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 (``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

     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
     (``POSIX.2''), and	should be used with caution in software	intended to be
     portable to other systems.	 Be warned also	that they are considered
     experimental 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.

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

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

     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	(``POSIX.2'') (such magic meanings occur only in obso-
     lete [``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

     A repetition operator (`?', `*', `+', or bounds) cannot follow another
     repetition	operator.  A repetition	operator cannot	begin an expression or
     subexpression 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.

     grep(1), re_format(7)

     IEEE Std 1003.2 (``POSIX.2''), sections 2.8 (Regular Expression Notation)
     and B.5 (C	Binding	for Regular Expression Matching).

     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

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

     This is an	alpha release with known defects.  Please report problems.

     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
     massively 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 (``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	9.2			March 20, 1994			   FreeBSD 9.2


Want to link to this manual page? Use this URL:

home | help