/* Void Main's man pages */
{ phpMan } else { main(); }
REGCOMP(3P) POSIX Programmer's Manual REGCOMP(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.
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
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 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 sub-
string.
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 IEEE Std 1003.1-2001, Chapter 9, Regular Expressions.)
REG_NOSUB
Report only success/fail in regexec().
REG_NEWLINE
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 Regu-
lar 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 subexpres-
sions (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 paren-
thesized 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 expres-
sion. Unused elements of pmatch up to pmatch[ nmatch-1] shall be filled with -1. If there are more than nmatch subexpres-
sions 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 regu-
lar 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 success-
ful 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 imme-
diately 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 unde-
fined.
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 IEEE Std 1003.1-2001, Chapter 9, Regular Expressions).
2. 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.
3. A dollar sign ( '$' ) in pattern, when used to specify expression anchoring, shall match the zero-length string imme-
diately 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:
REG_NOMATCH
regexec() failed to match.
REG_BADPAT
Invalid regular expression.
REG_ECOLLATE
Invalid collating element referenced.
REG_ECTYPE
Invalid character class type referenced.
REG_EESCAPE
Trailing '\' in pattern.
REG_ESUBREG
Number in "\digit" invalid or in error.
REG_EBRACK
"[]" imbalance.
REG_EPAREN
"\(\)" or "()" imbalance.
REG_EBRACE
"\{\}" imbalance.
REG_BADBR
Content of "\{\}" invalid: not a number, number too large, more than two numbers, first larger than second.
REG_ERANGE
Invalid endpoint in range expression.
REG_ESPACE
Out of memory.
REG_BADRPT
'?', '*', 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 imple-
mentations.
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 buf-
fer 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 IEEE Std 1003.1-2001, 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 applica-
tion, 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 appli-
cations, 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 applica-
tion 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 flexibil-
ity 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 implementa-
tions 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 han-
dler, 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 neg-
ative 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 con-
tains 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 subse-
quent 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 gen-
erated string. The scheme used by strerror() in the ISO C standard was considered unacceptable since it creates difficul-
ties 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 charac-
ter 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 spec-
ify 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(), Shell and Utilities volume of IEEE Std 1003.1-2001, Section 2.13, Pattern Matching Notation, Base Def-
initions volume of IEEE Std 1003.1-2001, Chapter 9, Regular Expressions, <regex.h>, <sys/types.h>
COPYRIGHT
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, Copy-
right (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 http://www.open-
group.org/unix/online.html .
IEEE/The Open Group 2003 REGCOMP(3P)
