regerror(3p) — Linux manual page


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

PROLOG         top

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

NAME         top

       regcomp, regerror, regexec, regfree — regular expression matching

SYNOPSIS         top

       #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         top

       These functions interpret basic and extended regular expressions as
       described in the Base Definitions volume of POSIX.1‐2008, 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.1‐2008, 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)


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

        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.1‐2008, Chapter 9, Regular

        2. A <circumflex> ('^') in pattern, when used to specify expression
           anchoring (see the Base Definitions volume of POSIX.1‐2008,
           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

       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         top

       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

       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         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top

           #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.

           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);
               if (status != 0) {
                   return(0);      /* Report error. */

       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);


       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.1‐2008, Section 2.13, Pattern Matching Notation, use the
       fnmatch() function.

RATIONALE         top

       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

       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

       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

       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

       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.1‐2008, Section 2.13, Pattern Matching Notation
       instead of REs. This service is now provided by the fnmatch()

       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.



SEE ALSO         top

       fnmatch(3p), glob(3p)

       The Base Definitions volume of POSIX.1‐2008, Chapter 9, Regular
       Expressions, regex.h(0p), sys_types.h(0p)

       The Shell and Utilities volume of POSIX.1‐2008, Section 2.13, Pattern
       Matching Notation

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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 .

       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 .

IEEE/The Open Group                 2013                         REGCOMP(3P)