NAME
regcomp
, regexec
,
regerror
, regfree
—
regular expression routines
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 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 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 (“POSIX.2”), and
should be used with caution in software intended to be portable to other
systems.
REG_EXTENDED
andREG_NOSPEC
may not be used in the same call toregcomp
(). 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 (“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 underREG_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 underREG_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 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 ‘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 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 the 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 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 extensions, 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
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 (“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 (“POSIX.2”) (such magic meanings occur only in obsolete 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
‘|
’.
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.
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
SEE ALSO
IEEE Std 1003.2 (“POSIX.2”), sections 2.8 (Regular Expression Notation) and B.5 (C Binding for Regular Expression Matching).
HISTORY
Predecessors to regcomp
() and
regexec
(), regcmp
() and
regex
(), first appeared in the Programmer's
Workbench (PWB/UNIX).
The present functions were originally written by Henry Spencer and 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 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 enormous REs or by multiply nested bounded repetitions, are probably not handled 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.