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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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REGCOMP(3P) POSIX Programmer's Manual REGCOMP(3P)
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,
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);
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().
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.
*/
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);
}
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.
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.
None.
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
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(3P)
Pages that refer to this page: regex.h(0p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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