REGCOMP(3C) Standard C Library Functions REGCOMP(3C)


regcomp, regexec, regerror, regfree - regular-expression library


Standard C Library (libc, -lc)


#include <regex.h>

regcomp(regex_t *restrict preg, const char *restrict pattern, int cflags);

regexec(const regex_t *restrict preg, const char *restrict string,
size_t nmatch, regmatch_t pmatch[restrict], int eflags);

regerror(int errcode, const regex_t *restrict preg, char *restrict errbuf,
size_t errbuf_size);

regfree(regex_t *preg);


These routines implement IEEE Std 1003.2 ("POSIX.2") regular expressions;
see regex(7). The regcomp() function compiles an RE written as a string
into an internal form, regexec() matches that internal form against a
string and reports results, regerror() transforms error codes from either
into human-readable messages, and regfree() frees any dynamically-allocated
storage used by the internal form of an RE.

The header <regex.h> declares two structure types, regex_t and regmatch_t,
the former for compiled internal forms and the latter for match reporting.
It also declares the four functions, a type regoff_t, and a number of
constants with names starting with "REG_".

The regcomp() function compiles the regular expression contained in the
pattern string, subject to the flags in cflags, and places the results in
the regex_t structure pointed to by preg. The cflags argument is the
bitwise OR of zero or more of the following flags:

REG_EXTENDED Compile extended regular expressions (EREs), rather than the
basic regular expressions (BREs) that are the default.

REG_BASIC This is a synonym for 0, provided as a counterpart to
REG_EXTENDED to improve readability.

REG_NOSPEC Compile with recognition of all special characters turned
off. All characters are thus considered ordinary, so the RE
is a literal string. This is an extension, compatible with
but not specified by IEEE Std 1003.2 ("POSIX.2"), and should
be used with caution in software intended to be portable to
other systems. REG_EXTENDED and REG_NOSPEC may not be used
in the same call to regcomp().

REG_ICASE Compile for matching that ignores upper/lower case
distinctions. See regex(7).

REG_NOSUB Compile for matching that need only report success or
failure, not what was matched.

REG_NEWLINE Compile for newline-sensitive matching. By default, newline
is a completely ordinary character with no special meaning in
either REs or strings. With this flag, "[^" bracket
expressions and "." never match newline, a "^" anchor matches
the null string after any newline in the string in addition
to its normal function, and the "$" anchor matches the null
string before any newline in the string in addition to its
normal function.

REG_PEND The regular expression ends, not at the first NUL, but just
before the character pointed to by the re_endp member of the
structure pointed to by preg. The re_endp member is of type
const char *. This flag permits inclusion of NULs in the RE;
they are considered ordinary characters. This is an
extension, compatible with but not specified by IEEE Std
1003.2 ("POSIX.2"), and should be used with caution in
software intended to be portable to other systems.

When successful, regcomp() returns 0 and fills in the structure pointed to
by preg. One member of that structure (other than re_endp) is publicized:
re_nsub, of type size_t, contains the number of parenthesized
subexpressions within the RE (except that the value of this member is
undefined if the REG_NOSUB flag was used).

The regexec() function matches the compiled RE pointed to by preg against
the string, subject to the flags in eflags, and reports results using
nmatch, pmatch, and the returned value. The RE must have been compiled by
a previous invocation of regcomp(). The compiled form is not altered
during execution of regexec(), so a single compiled RE can be used
simultaneously by multiple threads.

By default, the NUL-terminated string pointed to by string is considered to
be the text of an entire line, minus any terminating newline. The eflags
argument is the bitwise OR of zero or more of the following flags:

REG_NOTBOL The first character of the string is treated as the
continuation of a line. This means that the anchors "^",
"[[:<:]]", and "\<" do not match before it; but see
REG_STARTEND below. This does not affect the behavior of
newlines under REG_NEWLINE.

REG_NOTEOL The NUL terminating the string does not end a line, so the
"$" anchor does not match before it. This does not affect
the behavior of newlines under REG_NEWLINE.

REG_STARTEND The string is considered to start at string + pmatch[0].rm_so
and to end before the byte located at string +
pmatch[0].rm_eo, regardless of the value of nmatch. See
below for the definition of pmatch and nmatch. This is an
extension, compatible with but not specified by IEEE Std
1003.2 ("POSIX.2"), and should be used with caution in
software intended to be portable to other systems.

Without REG_NOTBOL, the position rm_so is considered the
beginning of a line, such that "^" matches before it, and the
beginning of a word if there is a word character at this
position, such that "[[:<:]]" and "\<" match before it.

With REG_NOTBOL, the character at position rm_so is treated
as the continuation of a line, and if rm_so is greater than
0, the preceding character is taken into consideration. If
the preceding character is a newline and the regular
expression was compiled with REG_NEWLINE, "^" matches before
the string; if the preceding character is not a word
character but the string starts with a word character,
"[[:<:]]" and "\<" match before the string.

See regex(7) for a discussion of what is matched in situations where an RE
or a portion thereof could match any of several substrings of string.

If REG_NOSUB was specified in the compilation of the RE, or if nmatch is 0,
regexec() ignores the pmatch argument (but see below for the case where
REG_STARTEND is specified). Otherwise, pmatch points to an array of nmatch
structures of type regmatch_t. Such a structure has at least the members
rm_so and rm_eo, both of type regoff_t (a signed arithmetic type at least
as large as an off_t and a ssize_t), containing respectively the offset of
the first character of a substring and the offset of the first character
after the end of the substring. Offsets are measured from the beginning of
the string argument given to regexec(). An empty substring is denoted by
equal offsets, both indicating the character following the empty substring.

The 0th member of the pmatch array is filled in to indicate what substring
of string was matched by the entire RE. Remaining members report what
substring was matched by parenthesized subexpressions within the RE; member
i reports subexpression i, with subexpressions counted (starting at 1) by
the order of their opening parentheses in the RE, left to right. Unused
entries in the array (corresponding either to subexpressions that did not
participate in the match at all, or to subexpressions that do not exist in
the RE (that is, i > preg->re_nsub)) have both rm_so and rm_eo set to -1.
If a subexpression participated in the match several times, the reported
substring is the last one it matched. (Note, as an example in particular,
that when the RE "(b*)+" matches "bbb", the parenthesized subexpression
matches each of the three `b's and then an infinite number of empty strings
following the last "b", so the reported substring is one of the empties.)

If REG_STARTEND is specified, pmatch must point to at least one regmatch_t
(even if nmatch is 0 or REG_NOSUB was specified), to hold the input offsets
for REG_STARTEND. Use for output is still entirely controlled by nmatch;
if nmatch is 0 or REG_NOSUB was specified, the value of pmatch[0] will not
be changed by a successful regexec().

The regerror() function maps a non-zero errcode from either regcomp() or
regexec() to a human-readable, printable message. If preg is non-NULL, the
error code should have arisen from use of the regex_t pointed to by preg,
and if the error code came from regcomp(), it should have been the result
from the most recent regcomp() using that regex_t. The (regerror() may be
able to supply a more detailed message using information from the regex_t.)
The regerror() function places the NUL-terminated message into the buffer
pointed to by errbuf, limiting the length (including the NUL) to at most
errbuf_size bytes. If the whole message will not fit, as much of it as
will fit before the terminating NUL is supplied. In any case, the returned
value is the size of buffer needed to hold the whole message (including
terminating NUL). If errbuf_size is 0, errbuf is ignored but the return
value is still correct.

If the errcode given to regerror() is first ORed with REG_ITOA, the
"message" that results is the printable name of the error code, e.g.
"REG_NOMATCH", rather than an explanation thereof. If errcode is REG_ATOI,
then preg shall be non-NULL and the re_endp member of the structure it
points to must point to the printable name of an error code; in this case,
the result in errbuf is the decimal digits of the numeric value of the
error code (0 if the name is not recognized). REG_ITOA and REG_ATOI are
intended primarily as debugging facilities; they are extensions, compatible
with but not specified by IEEE Std 1003.2 ("POSIX.2"), and should be used
with caution in software intended to be portable to other systems.

The regfree() function frees any dynamically-allocated storage associated
with the compiled RE pointed to by preg. The remaining regex_t is no
longer a valid compiled RE and the effect of supplying it to regexec() or
regerror() is undefined.


There are a number of decisions that IEEE Std 1003.2 ("POSIX.2") leaves up
to the implementor, either by explicitly saying "undefined" or by virtue of
them being forbidden by the RE grammar. This implementation treats them as

There is no particular limit on the length of REs, except insofar as memory
is limited. Memory usage is approximately linear in RE size, and largely
insensitive to RE complexity, except for bounded repetitions.

A backslashed character other than one specifically given a magic meaning
by IEEE Std 1003.2 ("POSIX.2") (such magic meanings occur only in BREs) is
taken as an ordinary character.

Any unmatched "[" is a REG_EBRACK error.

Equivalence classes cannot begin or end bracket-expression ranges. The
endpoint of one range cannot begin another.

RE_DUP_MAX, the limit on repetition counts in bounded repetitions, is 255.

A repetition operator ("?", "*", "+", or bounds) cannot follow another
repetition operator. A repetition operator cannot begin an expression or
subexpression or follow "^" or "|".

"|" cannot appear first or last in a (sub)expression or after another "|",
i.e., an operand of "|" cannot be an empty subexpression. An empty
parenthesized subexpression, "()", is legal and matches an empty
(sub)string. An empty string is not a legal RE.

A "{" followed by a digit is considered the beginning of bounds for a
bounded repetition, which must then follow the syntax for bounds. A "{"
not followed by a digit is considered an ordinary character.

"^" and "$" beginning and ending subexpressions in BREs are anchors, not
ordinary characters.


On successful completion, the regcomp() function returns 0. Otherwise, it
returns an integer value indicating an error as described in <regex.h>, and
the content of preg is undefined.

On successful completion, the regexec() function returns 0. Otherwise it
returns REG_NOMATCH to indicate no match, or REG_ENOSYS to indicate that
the function is not supported.

Upon successful completion, the regerror() function returns the number of
bytes needed to hold the entire generated string. Otherwise, it returns 0
to indicate that the function is not implemented.

The regfree() function returns no value.

The following constants are defined as error return values:

REG_NOMATCH The regexec() function 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_ENOSYS The function is not supported.
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.


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
(see malloc(3C)). Alternately, it could allocate a fixed, static buffer
that is big enough to hold most strings, and then use malloc() allocate a
larger buffer if it finds that this is too small.


Matching string against the extended regular expression in pattern.

#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

(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);


No errors are defined.





MT-Safe with exceptions

The regcomp() function can be used safely in a multithreaded application as
long as setlocale(3C) is not being called to change the locale.


attributes(7), regex(7), standards(7)

IEEE Std 1003.2 ("POSIX.2"), sections 2.8 (Regular Expression Notation) and
B.5 (C Binding for Regular Expression Matching).

OmniOS June 14, 2017 OmniOS