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RocketLink!--> Man page versions: OpenBSD FreeBSD Others

[IEEE Std1003.2-1992 (``POSIX.2'').]

REGEX(3)                  OpenBSD Programmer's Manual                 REGEX(3)

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

     #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);

     These routines implement POSIX 1003.2 regular expressions (``REs''); see
     re_format(7).  regcomp() compiles an RE written as a string into an in-
     ternal form, regexec() matches that internal form against a string and
     reports results, regerror() transforms error codes from either into hu-
     man-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 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, 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 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 mem-
                     ber is of type const char *. This flag permits inclusion
                     of NULs in the RE; they are considered ordinary charac-
                     ters.  This is an extension, compatible with but not
                     specified by POSIX 1003.2, and should be used with cau-
                     tion in software intended to be portable to other sys-

     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 un-
     defined 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 in-
     vocation of regcomp().  The compiled form is not altered during execution
     of regexec(), so a single compiled RE can be used simultaneously by mul-
     tiple threads.

     By default, the null-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_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

     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 parenthe-
     sized subexpression matches each of the three `bs' 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 null-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 the terminat-
     ing NUL).  If errbuf_size is 0, errbuf is ignored but the return value is
     still correct.

     If the errcode given to regerror() is first OR'ed 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 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 pos-

     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 1003.2 leaves up to the implementor,
     either by explicitly saying ``undefined'' or by virtue of them being for-
     bidden 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 1003.2 (such magic meanings occur only in obsolete 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 repeti-
     tion operator.  A repetition operator cannot begin an expression or
     subexpression or follow `^' or `|'.

     A `|' 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)

     POSIX 1003.2, sections 2.8 (Regular Expression Notation) and B.5 (C Bind-
     ing 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.

     There is one known functionality bug.  The implementation of internation-
     alization 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

     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.  A 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 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.

OpenBSD 2.6                     March 20, 1994                               5

Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.

(Corrections, notes, and links courtesy of RocketAware.com)

[Detailed Topics]
FreeBSD Sources for regex(3) functions
OpenBSD sources for regex(3)

[Overview Topics]

Up to: NUL terminated String Comparison and Search - covers functions for comparing strings, finding characters within strings, et al.

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[IEEE Std1003.2-1992 (``POSIX.2'').]

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