These routines implement IEEE Std 1003.2-1992 (“POSIX.2”) regular expressions (``RE''s); see
re_format(7).
regcomp() 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_''.
regcomp() 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.
cflags is the bitwise OR of zero or more of the following flags:
REG_EXTENDED
Compile modern (``extended'') REs, rather than the obsolete (``basic'') REs 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 1003.2-1992 (“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
re_format(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 `$' anchor matches the null string before any newline in the string in addition to its normal function.
REG_PEND
The regular expression ends, not at the first NUL, but just before the character pointed to by the re_endp member of the structure pointed to by preg. The re_endp member is of type const char *. This flag permits inclusion of NULs in the RE; they are considered ordinary characters. This is an extension, compatible with but not specified by IEEE Std 1003.2-1992 (“POSIX.2”), and should be used with caution in software intended to be portable to other systems.
When successful,
regcomp() returns 0 and fills in the structure pointed to by
preg. One member of that structure (other than
re_endp) is publicized:
re_nsub, of type
size_t, contains the number of parenthesized subexpressions within the RE (except that the value of this member is undefined if the
REG_NOSUB flag was used). If
regcomp() fails, it returns a non-zero error code; see
DIAGNOSTICS.
regexec() matches the compiled RE pointed to by
preg against the
string, subject to the flags in
eflags, and reports results using
nmatch,
pmatch, and the returned value. The RE must have been compiled by a previous invocation of
regcomp(). The compiled form is not altered during execution of
regexec(), so a single compiled RE can be used simultaneously by multiple threads.
By default, the NUL-terminated string pointed to by
string is considered to be the text of an entire line, minus any terminating newline. The
eflags argument is the bitwise OR of zero or more of the following flags:
REG_NOTBOL
The first character of the string is not the beginning of a line, so the `^' anchor should not match before it. This does not affect the behavior of newlines under REG_NEWLINE.
REG_NOTEOL
The NUL terminating the string does not end a line, so the `$' anchor should 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 have a terminating NUL located at string + pmatch[0].rm_eo (there need not actually be a NUL at that location), 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 1003.2-1992 (“POSIX.2”), and should be used with caution in software intended to be portable to other systems. Note that a non-zero rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location of the string, not how it is matched.
See
re_format(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.
Normally,
regexec() returns 0 for success and the non-zero code
REG_NOMATCH for failure. Other non-zero error codes may be returned in exceptional situations; see
DIAGNOSTICS.
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() 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. (
regerror() may be able to supply a more detailed message using information from the
regex_t.)
regerror() 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 won't 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 1003.2-1992 (“POSIX.2”), and should be used with caution in software intended to be portable to other systems. Be warned also that they are considered experimental and changes are possible.
regfree() 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.
None of these functions references global variables except for tables of constants; all are safe for use from multiple threads if the arguments are safe.