Tribblix: manual page: regcomp.3c

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

NAME

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

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#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);

DESCRIPTION

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_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).

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_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().

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.

IMPLEMENTATION NOTES

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.

RETURN VALUES

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.

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 (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.

EXAMPLES

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);
}

ERRORS

No errors are defined.

CODE SET INDEPENDENCE

Enabled

INTERFACE STABILITY

Standard

MT-LEVEL
MT-Safe with exceptions

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.