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

NAME
     regcomp, regexec, regerror, regfree — 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[restrict], int eflags);

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

     void
     regfree(regex_t *preg);

DESCRIPTION
     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 ‘^’
		   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 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 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
     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().

     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 informa‐
     tion from the regex_t.)  The regerror() function places the NUL-termi‐
     nated 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 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.

     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.

IMPLEMENTATION CHOICES
     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
     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 (“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
     255.

     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.

DIAGNOSTICS
     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)

SEE ALSO
     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).

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

BUGS
     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
     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 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				August 17, 2005				   BSD
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