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REGEX(3)							      REGEX(3)

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

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

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

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

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

       void regfree(regex_t *preg);

DESCRIPTION
       These  routines	implement  POSIX 1003.2 regular expressions (``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 dynamically-allocated
       storage used by the internal form of an RE.

       The header <regex.h> declares two structure  types,  regex_t  and  reg‐
       match_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 ordinary, so the
		     ``RE'' is a literal string.  This is an  extension,  com‐
		     patible  with  but	 not  specified	 by  POSIX 1003.2, 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, 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  mem‐
		     ber  of  the  structure  pointed to by preg.  The re_endp
		     member is of type const char *.  This flag permits inclu‐
		     sion  of  NULs  in	 the  RE; they are considered ordinary
		     characters.  This is an extension,	 compatible  with  but
		     not  specified  by	 POSIX 1003.2, and should be used with
		     caution in software intended to be portable to other sys‐
		     tems.

       When  successful,  regcomp returns 0 and fills in the structure pointed
       to by preg.  One member of that structure (other than re_endp) is  pub‐
       licized:	 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 dur‐
       ing execution of regexec, so a single compiled RE can be used  simulta‐
       neously by multiple threads.

       By  default,  the NUL-terminated string pointed to by string is consid‐
       ered 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
		     POSIX 1003.2, 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 following 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 follow‐
       ing the last `b', so the reported substring is one of the empties.)

       If REG_STARTEND is specified, pmatch must point to at  least  one  reg‐
       match_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 extensions, compatible with but not specified by POSIX 1003.2,
       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 argu‐
       ments are safe.

IMPLEMENTATION CHOICES
       There are a number of decisions that 1003.2 leaves up to the  implemen‐
       tor,  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
       memory  is  limited.   Memory usage is approximately linear in RE size,
       and largely insensitive to RE complexity, except	 for  bounded  repeti‐
       tions.	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 mean‐
       ing  by	1003.2 (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	 repe‐
       tition  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.

SEE ALSO
       grep(1), re_format(7)

       POSIX 1003.2, sections 2.8 (Regular Expression  Notation)  and  B.5  (C
       Binding for Regular Expression Matching).

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

HISTORY
       Originally  written by Henry Spencer at University of Toronto.  Altered
       for inclusion in the 4.4BSD distribution.

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

       There is one known functionality bug.  The implementation  of  interna‐
       tionalization  is  incomplete:  the  locale is always assumed to be the
       default one of 1003.2, and only the collating  elements	etc.  of  that
       locale are available.

       The  back-reference code is subtle and doubts linger about its correct‐
       ness 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
       enormous	 REs  or  by multiply nested bounded repetitions, are probably
       not handled well.

       Due to a mistake in 1003.2, 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,
       behavior 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.

				March 16, 1994			      REGEX(3)
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