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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(3POSIX)