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regcomp, regexec, regerror, regfree - regular-expression
library
SYNOPSIS
#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);
DESCRIPTION
These routines implement POSIX 1003.2 regular expressions
(``RE''s); see re_format(7). Regcomp compiles an RE writ-
ten 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 coun-
terpart to REG_EXTENDED to improve readabil-
ity.
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, com-
patible with but not specified by POSIX
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_for-
mat(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 new-
line, a `^' anchor matches the null string
after any newline in the string in addition
to its normal function, and the `$' anchor
matches the null string before any newline
in the string in addition to its normal
function.
REG_PEND The regular expression ends, not at the
first NUL, but just before the character
pointed to by the re_endp member of the
structure pointed to by preg. The re_endp
member is of type const char *. This flag
permits inclusion of NULs in the RE; they
are considered ordinary characters. This is
an extension, compatible with but not speci-
fied by POSIX 1003.2, and should be used
with caution in software intended to be
portable to other systems.
When successful, regcomp returns 0 and fills in the struc-
ture pointed to by preg. One member of that structure
(other than re_endp) is publicized: re_nsub, of type
size_t, contains the number of parenthesized subexpres-
sions within the RE (except that the value of this member
is undefined if the REG_NOSUB flag was used). If regcomp
fails, it returns a non-zero error code; see DIAGNOSTICS.
Regexec matches the compiled RE pointed to by preg against
the string, subject to the flags in eflags, and reports
results using nmatch, pmatch, and the returned value. The
RE must have been compiled by a previous invocation of
regcomp. The compiled form is not altered during execu-
tion of regexec, so a single compiled RE can be used
simultaneously by multiple threads.
By default, the NUL-terminated string pointed to by string
is considered to be the text of an entire line, minus any
REG_NOTBOL The first character of the string is not the
beginning of a line, so the `^' anchor
should not match before it. This does not
affect the behavior of newlines under
REG_NEWLINE.
REG_NOTEOL The NUL terminating the string does not end
a line, so the `$' anchor should not match
before it. This does not affect the behav-
ior of newlines under REG_NEWLINE.
REG_STARTEND The string is considered to start at
string + pmatch[0].rm_so and to have a ter-
minating 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 defini-
tion of pmatch and nmatch. This is an
extension, compatible with but not specified
by POSIX 1003.2, 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_STAR-
TEND 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 DIAGNOS-
TICS.
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 speci-
fied). 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 off-
sets, both indicating the character following the empty
substring.
The 0th member of the pmatch array is filled in to indi-
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 par-
ticipate 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 par-
ticipated in the match several times, the reported sub-
string 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 emp-
ties.)
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 cor-
rect.
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 POSIX
1003.2, and should be used with caution in software
sible.
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 multi-
ple threads if the arguments are safe.
IMPLEMENTATION CHOICES
There are a number of decisions that 1003.2 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 approxi-
mately 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 1003.2 (such magic meanings occur only
in obsolete [``basic''] REs) is taken as an ordinary char-
acter.
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 rep-
etitions, 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 subexpres-
sion, `()', 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
`^' and `$' beginning and ending subexpressions in obso-
lete (``basic'') REs are anchors, not ordinary characters.
SEE ALSO
grep(1), re_format(7)
POSIX 1003.2, sections 2.8 (Regular Expression Notation)
and B.5 (C Binding for Regular Expression Matching).
DIAGNOSTICS
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
HISTORY
Originally written by Henry Spencer. Altered for inclu-
sion in the 4.4BSD distribution.
BUGS
This is an alpha release with known defects. Please
report problems.
There is one known functionality bug. The implementation
of internationalization is incomplete: the locale is
always assumed to be the default one of 1003.2, 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 exceed-
ing 1 is worse. Regexec is largely insensitive to RE com-
plexity except that back references are massively expen-
sive. RE length does matter; in particular, there is a
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 (even-
tually) 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 multi-
ply nested bounded repetitions, are probably not handled
well.
Due to a mistake in 1003.2, things like `a)b' are legal
REs because `)' is a special character only in the pres-
ence 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.
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