regexec (3p) - Linux Manuals

regexec: regular expression matching


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.


regcomp, regerror, regexec, regfree - regular expression matching


#include <regex.h>

int regcomp(regex_t *restrict preg, const char *restrict pattern,
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,
nmatch, regmatch_t pmatch[restrict], int eflags);
void regfree(regex_t *


These functions interpret basic and extended regular expressions as described in the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 9, Regular Expressions.

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

Member TypeMember Name    Description    
size_tre_nsub    Number of parenthesized subexpressions.    

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

Member TypeMember Name     Description   
regoff_trm_so     Byte offset from start of string to start of substring.   
regoff_trm_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:

Use Extended Regular Expressions.
Ignore case in match. (See the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 9, Regular Expressions.)
Report only success/fail in regexec().
Change the handling of <newline>s, 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:

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.
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:

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.

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.

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.

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.

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:

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 IEEE Std 1003.1-2001, Chapter 9, Regular Expressions).

A circumflex ( '^' ) in pattern, when used to specify expression anchoring (see the Base Definitions volume of IEEE Std 1003.1-2001, 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.

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 error return values:

regexec() failed to match.
Invalid regular expression.
Invalid collating element referenced.
Invalid character class type referenced.
Trailing '\' in pattern.
Number in "\digit" invalid or in error.
"[]" imbalance.
"\(\)" or "()" imbalance.
"\{\}" imbalance.
Content of "\{\}" invalid: not a number, number too large, more than two numbers, first larger than second.
Invalid endpoint in range expression.
Out of memory.
'?', '*', or '+' not preceded by valid regular expression.

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().


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.


No errors are defined.

The following sections are informative.


#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 IEEE Std 1003.1-2001, Section 2.13, Pattern Matching Notation, use the fnmatch() function.


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 writer 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>s 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. Since this is a new interface, there should be no impact on historical implementations or applications, and offsets should be just as easy to use as pointers. The change to offsets was made to facilitate future extensions in which the string to be searched is presented to regexec() in blocks, allowing a string to be searched that is not all in memory at once.

The type regoff_t is used for the elements of pmatch[] to ensure that the application can represent either the largest possible array in memory (important for an application conforming to the Shell and Utilities volume of IEEE Std 1003.1-2001) or the largest possible file (important for an application using the extension where a file is searched in chunks).

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 IEEE Std 1003.1-2001, 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 IEEE Std 1003.1-2001 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". IEEE Std 1003.1-2001, 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". IEEE Std 1003.1-2001 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.




Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .


fnmatch(), glob(), Shell and Utilities volume of IEEE Std 1003.1-2001, Section 2.13, Pattern Matching Notation, Base Definitions volume of IEEE Std 1003.1-2001, Chapter 9, Regular Expressions, <regex.h>, <sys/types.h>