| REGCOMP(3C) | Standard C Library Functions | REGCOMP(3C) |
regcomp, regexec,
regerror, regfree —
regular-expression library
Standard C Library (libc, -lc)
#include
<regex.h>
int
regcomp(regex_t *restrict preg,
const char *restrict pattern, int
cflags);
int
regexec(const regex_t *restrict
preg, const char *restrict string,
size_t nmatch, regmatch_t
pmatch[restrict], int eflags);
size_t
regerror(int errcode,
const regex_t *restrict preg, char
*restrict errbuf, size_t errbuf_size);
void
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 translation of an RE into the internal form
contained in a regex_t is inherently locale-specific;
changes to the locale in effect between
regcomp()
and subsequent calls to regexec() may result in
unexpected or undefined behavior.
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_".
regcomp()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_EXTENDEDREG_BASICREG_EXTENDED to improve readability.REG_NOSPECREG_EXTENDED and
REG_NOSPEC may not be used in the same call to
regcomp().REG_ICASEREG_NOSUBREG_NEWLINEREG_PENDWhen 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).
regexec()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. The locale in effect at the time
of regexec() must be the same as the one in effect
when the RE was compiled by regcomp().
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_NOTBOLREG_STARTEND below. This does not affect the
behavior of newlines under REG_NEWLINE.REG_NOTEOLREG_NEWLINE.REG_STARTENDWithout 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().
regerror()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.
regfree()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 follows.
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_NOMATCHregexec() function failed to match.REG_BADPATREG_ECOLLATEREG_ECTYPEREG_EESCAPEREG_ESUBREGREG_EBRACKREG_ENOSYSREG_EPARENREG_EBRACEREG_BADBRREG_ERANGEREG_ESPACEREG_BADRPTAn 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
*/
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);
}
No errors are defined.
The
regcomp()
function can be used safely in a multithreaded application as long as
setlocale(3C) or
uselocale(3C) are not being called
to change the locale.
attributes(7), locale(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).
| December 26, 2023 | OmniOS |