NAME

regex, 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[], 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 1003.2-1992 (“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:

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:

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.

IMPLEMENTATION CHOICES

There are a number of decisions that IEEE Std 1003.2-1992 (“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.

See re_format(7) for a discussion of the definition of case-independent matching.

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. See BUGS for one short RE using them that will run almost any system out of memory.

A backslashed character other than one specifically given a magic meaning by IEEE Std 1003.2-1992 (“POSIX.2”) (such magic meanings occur only in obsolete [``basic''] REs) 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 obsolete (``basic'') REs are anchors, not ordinary characters.

DIAGNOSTICS

Non-zero error codes from regcomp() and regexec() include the following:

HISTORY

Originally written by Henry Spencer. Altered for inclusion in the 4.4BSD distribution.

BUGS

There is one known functionality bug. The implementation of internationalization is incomplete: the locale is always assumed to be the default one of IEEE Std 1003.2-1992 (“POSIX.2”), and only the collating elements etc. of that locale are available.

The back-reference code is subtle and doubts linger about its correctness in complex cases.

regexec() performance is poor. This will improve with later releases. nmatch exceeding 0 is expensive; nmatch exceeding 1 is worse. regexec is largely insensitive to RE complexity except that back references are massively expensive. RE length does matter; in particular, there is a strong speed bonus for keeping RE length under about 30 characters, with most special characters counting roughly double.

regcomp() implements bounded repetitions by macro expansion, which is costly in time and space if counts are large or bounded repetitions are nested. An RE like, say, `((((a{1,100}){1,100}){1,100}){1,100}){1,100}' will (eventually) run almost any existing machine out of swap space.

There are suspected problems with response to obscure error conditions. Notably, certain kinds of internal overflow, produced only by truly enormous REs or by multiply nested bounded repetitions, are probably not handled well.

Due to a mistake in IEEE Std 1003.2-1992 (“POSIX.2”), things like `a)b' are legal REs because `)' is a special character only in the presence of a previous unmatched `('. This can't be fixed until the spec is fixed.

The standard's definition of back references is vague. For example, does `a$\(b$*\2\)*d' match `abbbd'? Until the standard is clarified, behavior in such cases should not be relied on.

The implementation of word-boundary matching is a bit of a kludge, and bugs may lurk in combinations of word-boundary matching and anchoring.

Manual browser: regex(3)