# regex(3)

    NAME
         regex, 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
         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,
                       compatible  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
                       distinctions.  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 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 POSIX
                       1003.2,  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
         publicized:   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 execution
         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  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 respectively 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
         substring  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
         controlled  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 arguments
         are safe.

    IMPLEMENTATION CHOICES
         There are a number of decisions that 1003.2 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
         memory  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 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
         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.

    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.  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
         internationalization  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 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
         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.