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REGEX(3)		 BSD Library Functions Manual		      REGEX(3)

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

     Standard C	Library	(libc, -lc)

     #include <regex.h>

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

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

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

     regfree(regex_t *preg);

     These routines implement IEEE Std 1003.2 ("POSIX.2") regular expressions
     ("RE"s); see re_format(7).	 The regcomp() function	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_".

     The regcomp() function 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.	The cflags argument 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, compatible
		   with	but not	specified by IEEE Std 1003.2 ("POSIX.2"), and
		   should be used with caution in software intended to be por-
		   table 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 `^' an-
		   chor	matches	the null string	after any newline in the
		   string in addition to its normal function, and the `$' an-
		   chor	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.

     REG_POSIX	   Compile only	IEEE Std 1003.2	("POSIX.2") compliant expres-
		   sions.  This	flag has no effect unless linking against
		   libregex.  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 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.

     The regexec() function 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 execution of regexec(), so a single	compiled RE can	be used	simul-
     taneously 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	treated	as the contin-
		   uation of a line.  This means that the anchors `^',
		   `[[:<:]]', and `\<' do not match before it; but see
		   REG_STARTEND	below.	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 does 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 end before the byte located at
		   string + pmatch[0].rm_eo, 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	cau-
		   tion	in software intended to	be portable to other systems.

		   Without REG_NOTBOL, the position rm_so is considered	the
		   beginning of	a line,	such that `^' matches before it, and
		   the beginning of a word if there is a word character	at
		   this	position, such that `[[:<:]]' and `\<' match before

		   With	REG_NOTBOL, the	character at position rm_so is treated
		   as the continuation of a line, and if rm_so is greater than
		   0, the preceding character is taken into consideration.  If
		   the preceding character is a	newline	and the	regular	ex-
		   pression was	compiled with REG_NEWLINE, `^' matches before
		   the string; if the preceding	character is not a word	char-
		   acter but the string	starts with a word character,
		   `[[:<:]]' and `\<' match before the string.

     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().

     The regerror() function 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.	The
     (regerror() may be	able to	supply a more detailed message using
     information from the regex_t.)  The regerror() function 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 will not 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 cor-

     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.

     The regfree() function 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.

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

     REG_NOMATCH   The regexec() function 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	   cannot happen - you found a bug
     REG_INVARG	   invalid argument, e.g. negative-length string
     REG_ILLSEQ	   illegal byte	sequence (bad multibyte	character)

     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).

     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.

     The regexec() function performance	is poor.  This will improve with later
     releases.	The nmatch argument exceeding 0	is expensive; nmatch exceeding
     1 is worse.  The regexec()	function is largely insensitive	to RE complex-
     ity 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.

     The regcomp() function implements bounded repetitions by macro expansion,
     which is costly in	time and space if counts are large or bounded repeti-
     tions are nested.	An RE like, say,
     `((((a{1,100}){1,100}){1,100}){1,100}){1,100}' will (eventually) run al-
     most 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 cannot 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.

     Word-boundary matching does not work properly in multibyte	locales.

BSD				April 15, 2017				   BSD


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