REGCOMP(3)                 POSIX Programmer's Manual                REGCOMP(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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

SYNOPSIS
       #include <regex.h>

       int regcomp(regex_t *restrict preg, const char *restrict pattern,
           int cflags);
       size_t regerror(int errcode, const regex_t *restrict preg,
           char *restrict errbuf, size_t errbuf__size);
       int regexec(const regex_t *restrict preg, const char *restrict string,
           size_t nmatch, regmatch_t pmatch[restrict], int eflags);
       void regfree(regex_t *preg);

DESCRIPTION
       These functions interpret basic and extended regular expressions as
       described in the Base Definitions volume of POSIX.1-2017, Chapter _9,
       Regular Expressions.

       The regex_t structure is defined in <regex.h> and contains at least the
       following member:

             +--------------+--------------+---------------------------+
             |Member Type   | Member Name  |        Description        |
             +--------------+--------------+---------------------------+
             |size_t        |re__nsub       | Number of parenthesized   |
             |              |              | subexpressions.           |
             +--------------+--------------+---------------------------+
       The regmatch_t structure is defined in <regex.h> and contains at least
       the following members:

             +--------------+--------------+---------------------------+
             |Member Type   | Member Name  |        Description        |
             +--------------+--------------+---------------------------+
             |regoff_t      |rm__so         | Byte offset from start of |
             |              |              | string to start of        |
             |              |              | substring.                |
             |regoff_t      |rm__eo         | Byte offset from start of |
             |              |              | string of the first       |
             |              |              | character after the end   |
             |              |              | of substring.             |
             +--------------+--------------+---------------------------+
       The regcomp() function shall compile the regular expression contained
       in the string pointed to by the pattern argument and place the results
       in the structure pointed to by preg.  The cflags argument is the
       bitwise-inclusive OR of zero or more of the following flags, which are
       defined in the <regex.h> header:

       REG_EXTENDED  Use Extended Regular Expressions.

       REG_ICASE     Ignore case in match (see the Base Definitions volume of
                     POSIX.1-2017, Chapter _9, Regular Expressions).

       REG_NOSUB     Report only success/fail in regexec().

       REG_NEWLINE   Change the handling of <newline> characters, as described
                     in the text.

       The default regular expression type for pattern is a Basic Regular
       Expression. The application can specify Extended Regular Expressions
       using the REG_EXTENDED cflags flag.

       If the REG_NOSUB flag was not set in cflags, then regcomp() shall set
       re__nsub to the number of parenthesized subexpressions (delimited by
       ""\\((\\))"" in basic regular expressions or ""(())"" in extended regular
       expressions) found in pattern.

       The regexec() function compares the null-terminated string specified by
       string with the compiled regular expression preg initialized by a
       previous call to regcomp().  If it finds a match, regexec() shall
       return 0; otherwise, it shall return non-zero indicating either no
       match or an error. The eflags argument is the bitwise-inclusive OR of
       zero or more of the following flags, which are defined in the <regex.h>
       header:

       REG_NOTBOL    The first character of the string pointed to by string is
                     not the beginning of the line. Therefore, the
                     <circumflex> character (''^^''), when taken as a special
                     character, shall not match the beginning of string.

       REG_NOTEOL    The last character of the string pointed to by string is
                     not the end of the line. Therefore, the <dollar-sign>
                     (''$$''), when taken as a special character, shall not match
                     the end of string.

       If nmatch is 0 or REG_NOSUB was set in the cflags argument to
       regcomp(), then regexec() shall ignore the pmatch argument. Otherwise,
       the application shall ensure that the pmatch argument points to an
       array with at least nmatch elements, and regexec() shall fill in the
       elements of that array with offsets of the substrings of string that
       correspond to the parenthesized subexpressions of pattern:
       pmatch[_i].rm__so shall be the byte offset of the beginning and
       pmatch[_i].rm__eo shall be one greater than the byte offset of the end of
       substring _i.  (Subexpression _i begins at the _ith matched open
       parenthesis, counting from 1.) Offsets in pmatch[0] identify the
       substring that corresponds to the entire regular expression. Unused
       elements of pmatch up to pmatch[nmatch-1] shall be filled with -1. If
       there are more than nmatch subexpressions in pattern (pattern itself
       counts as a subexpression), then regexec() shall still do the match,
       but shall record only the first nmatch substrings.

       When matching a basic or extended regular expression, any given
       parenthesized subexpression of pattern might participate in the match
       of several different substrings of string, or it might not match any
       substring even though the pattern as a whole did match. The following
       rules shall be used to determine which substrings to report in pmatch
       when matching regular expressions:

        1. If subexpression _i in a regular expression is not contained within
           another subexpression, and it participated in the match several
           times, then the byte offsets in pmatch[_i] shall delimit the last
           such match.

        2. If subexpression _i is not contained within another subexpression,
           and it did not participate in an otherwise successful match, the
           byte offsets in pmatch[_i] shall be -1. A subexpression does not
           participate in the match when:

           ''**'' or ""\\{{\\}}"" appears immediately after the subexpression in a
           basic regular expression, or ''**'', ''??'', or ""{{}}"" appears immediately
           after the subexpression in an extended regular expression, and the
           subexpression did not match (matched 0 times)

           or:

                  ''||'' is used in an extended regular expression to select this
                  subexpression or another, and the other subexpression
                  matched.

        3. If subexpression _i is contained within another subexpression _j, and
           _i is not contained within any other subexpression that is contained
           within _j, and a match of subexpression _j is reported in pmatch[_j],
           then the match or non-match of subexpression _i reported in
           pmatch[_i] shall be as described in 1. and 2. above, but within the
           substring reported in pmatch[_j] rather than the whole string. The
           offsets in pmatch[_i] are still relative to the start of string.

        4. If subexpression _i is contained in subexpression _j, and the byte
           offsets in pmatch[_j] are -1, then the pointers in pmatch[_i] shall
           also be -1.

        5. If subexpression _i matched a zero-length string, then both byte
           offsets in pmatch[_i] shall be the byte offset of the character or
           null terminator immediately following the zero-length string.

       If, when regexec() is called, the locale is different from when the
       regular expression was compiled, the result is undefined.

       If REG_NEWLINE is not set in cflags, then a <newline> in pattern or
       string shall be treated as an ordinary character. If REG_NEWLINE is
       set, then <newline> shall be treated as an ordinary character except as
       follows:

        1. A <newline> in string shall not be matched by a <period> outside a
           bracket expression or by any form of a non-matching list (see the
           Base Definitions volume of POSIX.1-2017, Chapter _9, Regular
           Expressions).

        2. A <circumflex> (''^^'') in pattern, when used to specify expression
           anchoring (see the Base Definitions volume of POSIX.1-2017, Section
           9.3.8, BRE Expression Anchoring), shall match the zero-length
           string immediately after a <newline> in string, regardless of the
           setting of REG_NOTBOL.

        3. A <dollar-sign> (''$$'') in pattern, when used to specify expression
           anchoring, shall match the zero-length string immediately before a
           <newline> in string, regardless of the setting of REG_NOTEOL.

       The regfree() function frees any memory allocated by regcomp()
       associated with preg.

       The following constants are defined as the minimum set of error return
       values, although other errors listed as implementation extensions in
       <regex.h> are possible:

       REG_BADBR     Content of ""\\{{\\}}"" invalid: not a number, number too
                     large, more than two numbers, first larger than second.

       REG_BADPAT    Invalid regular expression.

       REG_BADRPT    ''??'', ''**'', or ''++'' not preceded by valid regular
                     expression.

       REG_EBRACE    ""\\{{\\}}"" imbalance.

       REG_EBRACK    ""[[]]"" imbalance.

       REG_ECOLLATE  Invalid collating element referenced.

       REG_ECTYPE    Invalid character class type referenced.

       REG_EESCAPE   Trailing <backslash> character in pattern.

       REG_EPAREN    ""\\((\\))"" or ""(())"" imbalance.

       REG_ERANGE    Invalid endpoint in range expression.

       REG_ESPACE    Out of memory.

       REG_ESUBREG   Number in "\digit" invalid or in error.

       REG_NOMATCH   regexec() failed to match.

       If more than one error occurs in processing a function call, any one of
       the possible constants may be returned, as the order of detection is
       unspecified.

       The regerror() function provides a mapping from error codes returned by
       regcomp() and regexec() to unspecified printable strings. It generates
       a string corresponding to the value of the errcode argument, which the
       application shall ensure is the last non-zero value returned by
       regcomp() or regexec() with the given value of preg.  If errcode is not
       such a value, the content of the generated string is unspecified.

       If preg is a null pointer, but errcode is a value returned by a
       previous call to regexec() or regcomp(), the regerror() still generates
       an error string corresponding to the value of errcode, but it might not
       be as detailed under some implementations.

       If the errbuf__size argument is not 0, regerror() shall place the
       generated string into the buffer of size errbuf__size bytes pointed to
       by errbuf.  If the string (including the terminating null) cannot fit
       in the buffer, regerror() shall truncate the string and null-terminate
       the result.

       If errbuf__size is 0, regerror() shall ignore the errbuf argument, and
       return the size of the buffer needed to hold the generated string.

       If the preg argument to regexec() or regfree() is not a compiled
       regular expression returned by regcomp(), the result is undefined. A
       preg is no longer treated as a compiled regular expression after it is
       given to regfree().

RETURN VALUE
       Upon successful completion, the regcomp() function shall return 0.
       Otherwise, it shall return an integer value indicating an error as
       described in <regex.h>, and the content of preg is undefined. If a code
       is returned, the interpretation shall be as given in <regex.h>.

       If regcomp() detects an invalid RE, it may return REG_BADPAT, or it may
       return one of the error codes that more precisely describes the error.

       Upon successful completion, the regexec() function shall return 0.
       Otherwise, it shall return REG_NOMATCH to indicate no match.

       Upon successful completion, the regerror() function shall return the
       number of bytes needed to hold the entire generated string, including
       the null termination. If the return value is greater than errbuf__size,
       the string returned in the buffer pointed to by errbuf has been
       truncated.

       The regfree() function shall not return a value.

ERRORS
       No errors are defined.

       The following sections are informative.

EXAMPLES
           #include <regex.h>

           /*
            * Match string against the extended regular expression in
            * pattern, treating errors as no match.
            *
            * Return 1 for match, 0 for no match.
            */

           int
           match(const char *string, char *pattern)
           {
               int    status;
               regex_t    re;

               if (regcomp(&re, pattern, REG_EXTENDED|REG_NOSUB) != 0) {
                   return(0);      /* Report error. */
               }
               status = regexec(&re, string, (size_t) 0, NULL, 0);
               regfree(&re);
               if (status != 0) {
                   return(0);      /* Report error. */
               }
               return(1);
           }

       The following demonstrates how the REG_NOTBOL flag could be used with
       regexec() to find all substrings in a line that match a pattern
       supplied by a user.  (For simplicity of the example, very little error
       checking is done.)


           (void) regcomp (&re, pattern, 0);
           /* This call to regexec() finds the first match on the line. */
           error = regexec (&re, &buffer[0], 1, &pm, 0);
           while (error == 0) {  /* While matches found. */
               /* Substring found between pm.rm_so and pm.rm_eo. */
               /* This call to regexec() finds the next match. */
               error = regexec (&re, buffer + pm.rm_eo, 1, &pm, REG_NOTBOL);
           }

APPLICATION USAGE
       An application could use:


           regerror(code,preg,(char *)NULL,(size_t)0)

       to find out how big a buffer is needed for the generated string,
       malloc() a buffer to hold the string, and then call regerror() again to
       get the string. Alternatively, it could allocate a fixed, static buffer
       that is big enough to hold most strings, and then use malloc() to
       allocate a larger buffer if it finds that this is too small.

       To match a pattern as described in the Shell and Utilities volume of
       POSIX.1-2017, Section 2.13, Pattern Matching Notation, use the
       fnmatch() function.

RATIONALE
       The regexec() function must fill in all nmatch elements of pmatch,
       where nmatch and pmatch are supplied by the application, even if some
       elements of pmatch do not correspond to subexpressions in pattern.  The
       application developer should note that there is probably no reason for
       using a value of nmatch that is larger than preg->re__nsub+1.

       The REG_NEWLINE flag supports a use of RE matching that is needed in
       some applications like text editors. In such applications, the user
       supplies an RE asking the application to find a line that matches the
       given expression. An anchor in such an RE anchors at the beginning or
       end of any line. Such an application can pass a sequence of
       <newline>-separated lines to regexec() as a single long string and
       specify REG_NEWLINE to regcomp() to get the desired behavior. The
       application must ensure that there are no explicit <newline> characters
       in pattern if it wants to ensure that any match occurs entirely within
       a single line.

       The REG_NEWLINE flag affects the behavior of regexec(), but it is in
       the cflags parameter to regcomp() to allow flexibility of
       implementation. Some implementations will want to generate the same
       compiled RE in regcomp() regardless of the setting of REG_NEWLINE and
       have regexec() handle anchors differently based on the setting of the
       flag. Other implementations will generate different compiled REs based
       on the REG_NEWLINE.

       The REG_ICASE flag supports the operations taken by the grep -i option
       and the historical implementations of ex and vi.  Including this flag
       will make it easier for application code to be written that does the
       same thing as these utilities.

       The substrings reported in pmatch[] are defined using offsets from the
       start of the string rather than pointers. This allows type-safe access
       to both constant and non-constant strings.

       The type regoff_t is used for the elements of pmatch[] to ensure that
       the application can represent large arrays in memory (important for an
       application conforming to the Shell and Utilities volume of
       POSIX.1-2017).

       The 1992 edition of this standard required regoff_t to be at least as
       wide as off_t, to facilitate future extensions in which the string to
       be searched is taken from a file. However, these future extensions have
       not appeared.  The requirement rules out popular implementations with
       32-bit regoff_t and 64-bit off_t, so it has been removed.

       The standard developers rejected the inclusion of a regsub() function
       that would be used to do substitutions for a matched RE. While such a
       routine would be useful to some applications, its utility would be much
       more limited than the matching function described here. Both RE parsing
       and substitution are possible to implement without support other than
       that required by the ISO C standard, but matching is much more complex
       than substituting. The only difficult part of substitution, given the
       information supplied by regexec(), is finding the next character in a
       string when there can be multi-byte characters. That is a much larger
       issue, and one that needs a more general solution.

       The errno variable has not been used for error returns to avoid filling
       the errno name space for this feature.

       The interface is defined so that the matched substrings rm__sp and rm__ep
       are in a separate regmatch_t structure instead of in regex_t.  This
       allows a single compiled RE to be used simultaneously in several
       contexts; in main() and a signal handler, perhaps, or in multiple
       threads of lightweight processes. (The preg argument to regexec() is
       declared with type const, so the implementation is not permitted to use
       the structure to store intermediate results.) It also allows an
       application to request an arbitrary number of substrings from an RE.
       The number of subexpressions in the RE is reported in re__nsub in preg.
       With this change to regexec(), consideration was given to dropping the
       REG_NOSUB flag since the user can now specify this with a zero nmatch
       argument to regexec().  However, keeping REG_NOSUB allows an
       implementation to use a different (perhaps more efficient) algorithm if
       it knows in regcomp() that no subexpressions need be reported. The
       implementation is only required to fill in pmatch if nmatch is not zero
       and if REG_NOSUB is not specified. Note that the size_t type, as
       defined in the ISO C standard, is unsigned, so the description of
       regexec() does not need to address negative values of nmatch.

       REG_NOTBOL was added to allow an application to do repeated searches
       for the same pattern in a line. If the pattern contains a <circumflex>
       character that should match the beginning of a line, then the pattern
       should only match when matched against the beginning of the line.
       Without the REG_NOTBOL flag, the application could rewrite the
       expression for subsequent matches, but in the general case this would
       require parsing the expression. The need for REG_NOTEOL is not as
       clear; it was added for symmetry.

       The addition of the regerror() function addresses the historical need
       for conforming application programs to have access to error information
       more than ``Function failed to compile/match your RE for unknown
       reasons''.

       This interface provides for two different methods of dealing with error
       conditions. The specific error codes (REG_EBRACE, for example), defined
       in <regex.h>, allow an application to recover from an error if it is so
       able. Many applications, especially those that use patterns supplied by
       a user, will not try to deal with specific error cases, but will just
       use regerror() to obtain a human-readable error message to present to
       the user.

       The regerror() function uses a scheme similar to confstr() to deal with
       the problem of allocating memory to hold the generated string. The
       scheme used by strerror() in the ISO C standard was considered
       unacceptable since it creates difficulties for multi-threaded
       applications.

       The preg argument is provided to regerror() to allow an implementation
       to generate a more descriptive message than would be possible with
       errcode alone. An implementation might, for example, save the character
       offset of the offending character of the pattern in a field of preg,
       and then include that in the generated message string. The
       implementation may also ignore preg.

       A REG_FILENAME flag was considered, but omitted. This flag caused
       regexec() to match patterns as described in the Shell and Utilities
       volume of POSIX.1-2017, Section 2.13, Pattern Matching Notation instead
       of REs. This service is now provided by the fnmatch() function.

       Notice that there is a difference in philosophy between the
       ISO POSIX-2:1993 standard and POSIX.1-2008 in how to handle a ``bad''
       regular expression. The ISO POSIX-2:1993 standard says that many bad
       constructs ``produce undefined results'', or that ``the interpretation
       is undefined''. POSIX.1-2008, however, says that the interpretation of
       such REs is unspecified. The term ``undefined'' means that the action
       by the application is an error, of similar severity to passing a bad
       pointer to a function.

       The regcomp() and regexec() functions are required to accept any null-
       terminated string as the pattern argument. If the meaning of the string
       is ``undefined'', the behavior of the function is ``unspecified''.
       POSIX.1-2008 does not specify how the functions will interpret the
       pattern; they might return error codes, or they might do pattern
       matching in some completely unexpected way, but they should not do
       something like abort the process.

FUTURE DIRECTIONS
       None.

SEE ALSO
       fnmatch(), glob()

       The Base Definitions volume of POSIX.1-2017, Chapter _9, Regular
       Expressions, <regex.h>, <sys_types.h>

       The Shell and Utilities volume of POSIX.1-2017, Section 2.13, Pattern
       Matching Notation

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                           REGCOMP(3)
