regcomp(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.1‐2017,
       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

       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

       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


                  '|' 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

       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

       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 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         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.1‐2017, 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 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

       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

       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.1‐2017, Chapter 9, Regular
       Expressions, regex.h(0p), sys_types.h(0p)

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

COPYRIGHT         top

       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 .

       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               2017                       REGCOMP(3P)

Pages that refer to this page: regex.h(0p)