REGEX(3) BSD Programmer's Manual REGEX(3)
regcomp, regexec, regerror, regfree - regular expression routines
#include <sys/types.h>
#include <regex.h>
int
regcomp(regex_t *preg, const char *pattern, int cflags);
int
regexec(const regex_t *preg, const char *string, size_t nmatch,
regmatch_t pmatch[], int eflags);
size_t
regerror(int errcode, const regex_t *preg, char *errbuf,
size_t errbuf_size);
void
regfree(regex_t *preg);
These routines implement IEEE Std 1003.2 ("POSIX.2") regular expressions
("REs"); 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 allo-
cated 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 ("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 dis-
tinctions. 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, new-
line 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 '$'
anchor matches the null string before any newline in the
string in addition to its normal function.
REG_PEND The regular expression ends, not at the first NUL, but
just before the character pointed to by the re_endp
member of the structure pointed to by preg. The re_endp
member is of type const char *. This flag permits inclu-
sion of NULs in the RE; they are considered ordinary
characters. This is an extension, compatible with but not
specified by IEEE Std 1003.2 ("POSIX.2"), and should be
used with caution in software intended to be portable to
other systems.
When successful, regcomp() returns 0 and fills in the structure pointed
to by preg. One member of that structure (other than re_endp) is publi-
cized: re_nsub, of type size_t, contains the number of parenthesized su-
bexpressions within the RE (except that the value of this member is unde-
fined if the REG_NOSUB flag was used). If regcomp() fails, it returns a
non-zero error code; see DIAGNOSTICS.
regexec() matches the compiled RE pointed to by preg against the string,
subject to the flags in eflags, and reports results using nmatch, pmatch,
and the returned value. The RE must have been compiled by a previous in-
vocation of regcomp(). The compiled form is not altered during execution
of regexec(), so a single compiled RE can be used simultaneously by mul-
tiple threads.
By default, the NUL-terminated string pointed to by string is considered
to be the text of an entire line, minus any terminating newline. The
eflags argument is the bitwise OR of zero or more of the following flags:
REG_NOTBOL The first character of the string is not the beginning of
a line, so the '^' anchor should not match before it.
This does not affect the behavior of newlines under
REG_NEWLINE.
REG_NOTEOL The NUL terminating the string does not end a line, so
the '$' 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 ("POSIX.2"), and should be used with cau-
tion in software intended to be portable to other sys-
tems. 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 sub-
string 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 'bs' 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 con-
trolled 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 sup-
ply a more detailed message using information from the regex_t.) reger-
ror() 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 the terminat-
ing NUL). If errbuf_size is 0, errbuf is ignored but the return value is
still correct.
If the errcode given to regerror() is first OR'ed 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 exten-
sions, 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. 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 reger-
ror() 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.
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 vir-
tue 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 ("POSIX.2") (such magic meanings occur only in ob-
solete 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 repeti-
tion operator. A repetition operator cannot begin an expression or subex-
pression or follow '^' or '|'.
A '|' 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.
Non-zero error codes from regcomp() and regexec() include the following:
REG_NOMATCH regexec() failed to match
REG_BADPAT invalid regular expression
REG_ECOLLATE invalid collating element
REG_ECTYPE invalid character class
REG_EESCAPE \ applied to unescapable character
REG_ESUBREG invalid backreference number
REG_EBRACK brackets [ ] not balanced
REG_EPAREN parentheses ( ) not balanced
REG_EBRACE braces { } not balanced
REG_BADBR invalid repetition count(s) in { }
REG_ERANGE invalid character range in [ ]
REG_ESPACE ran out of memory
REG_BADRPT ?, *, or + operand invalid
REG_EMPTY empty (sub)expression
REG_ASSERT "can't happen" -you found a bug
REG_INVARG invalid argument, e.g., negative-length string
grep(1), re_format(7)
IEEE Std 1003.2 ("POSIX.2"), sections 2.8 (Regular Expression Notation)
and B.5 (C Binding for Regular Expression Matching).
Originally written by Henry Spencer. Altered for inclusion in the 4.4BSD
distribution.
This is an alpha release with known defects. Please report problems.
There is one known functionality bug. The implementation of internation-
alization is incomplete: the locale is always assumed to be the default
one of IEEE Std 1003.2 ("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. A 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 enor-
mous REs or by multiply nested bounded repetitions, are probably not han-
dled well.
Due to a mistake in IEEE Std 1003.2 ("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.
MirBSD #10-current March 20, 1994 4