REGEX(3) REGEX(3)
March 20, 1994
NAME
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 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 POSIX
1003.2, 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.
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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 `$' anchor matches the null string
before any newline in the string in addition to its
normal function.
REG_PEND The regular expression ends, not at the first NUL, but
just before the character pointed to by the re_endp
member of the structure pointed to by preg. The re_endp
member is of type const char *. This flag permits
inclusion of NULs in the RE; they are considered
ordinary characters. This is an extension, compatible
with but not specified by POSIX 1003.2, and should be
used with caution in software intended to be portable to
other systems.
When successful, regcomp returns 0 and fills in the structure pointed
to by preg. One member of that structure (other than re_endp) is
publicized: re_nsub, of type size_t, contains the number of
parenthesized subexpressions within the RE (except that the value of
this member is undefined if the REG_NOSUB flag was used). If regcomp
fails, it returns a non-zero error code; see DIAGNOSTICS.
Regexec matches the compiled RE pointed to by preg against the string,
subject to the flags in eflags, and reports results using nmatch,
pmatch, and the returned value. The RE must have been compiled by a
previous invocation of regcomp. The compiled form is not altered
during execution of regexec, so a single compiled RE can be used
simultaneously by multiple threads.
By default, the NUL-terminated string pointed to by string is
considered to be the text of an entire line, minus any terminating
newline. The eflags argument is the bitwise OR of zero or more of the
following flags:
REG_NOTBOL The first character of the string is not the beginning
of a line, so the `^' anchor should not match before it.
This does not affect the behavior of newlines under
REG_NEWLINE.
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REG_NOTEOL The NUL terminating the string does not end a line, so
the `$' 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 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_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.)
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March 20, 1994
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 POSIX
1003.2, 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 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.
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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 1003.2 (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.
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
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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 inclusion 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 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 1003.2, 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.
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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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