REGEX(3) | Library Functions Manual | REGEX(3) |
regcomp
,
regcomp_l
, regerror
,
regexec
, regfree
,
regncomp
, regncomp_l
,
regnexec
, regnwcomp
,
regnwcomp_l
, regnwexec
,
regwcomp
, regwcomp_l
,
regwexec
—
regular-expression library
#include <regex.h>
int
regcomp
(regex_t *restrict preg,
const char *restrict pattern, int
cflags);
size_t
regerror
(int errcode,
const regex_t *restrict preg, char
*restrict errbuf, size_t errbuf_size);
int
regexec
(const regex_t *restrict
preg, const char *restrict string,
size_t nmatch, regmatch_t
pmatch[restrict], int eflags);
void
regfree
(regex_t *preg);
(Non-portable
extensions)
int
regncomp
(regex_t *restrict preg,
const char *restrict pattern, size_t
len, int cflags);
int
regnexec
(const regex_t *restrict
preg, const char *restrict string,
size_t len, size_t nmatch,
regmatch_t pmatch[restrict], int
eflags);
int
regwcomp
(regex_t *restrict preg,
const wchar_t *restrict widepat, int
cflags);
int
regwexec
(const regex_t *restrict
preg, const wchar_t *restrict widestr,
size_t nmatch, regmatch_t
pmatch[restrict], int eflags);
int
regwncomp
(regex_t *restrict
preg, const wchar_t *restrict widepat,
size_t len, int cflags);
int
regwnexec
(const regex_t *restrict
preg, const wchar_t *restrict widestr,
size_t len, size_t nmatch,
regmatch_t pmatch[restrict], int
eflags);
#include <regex.h>
#include <xlocale.h>
int
regcomp_l
(regex_t *restrict
preg, const char *restrict pattern,
int cflags, locale_t
restrict);
int
regncomp_l
(regex_t *restrict
preg, const char *restrict pattern,
size_t len, int cflags,
locale_t restrict);
int
regwcomp_l
(regex_t *restrict
preg, const wchar_t *restrict widepat,
int cflags, locale_t
restrict);
int
regwncomp_l
(regex_t *restrict
preg, const wchar_t *restrict widepat,
size_t len, int cflags,
locale_t restrict);
These routines implement IEEE Std 1003.2
(“POSIX.2”) regular expressions (“RE”s);
see re_format(7). The
regcomp
()
function 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.
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_
”.
The
regcomp
()
function 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. The cflags argument is the
bitwise OR of zero or more of the following flags:
REG_EXTENDED
REG_BASIC
REG_EXTENDED
to improve readability.REG_NOSPEC
REG_EXTENDED
and
REG_NOSPEC
may not be used in the same call to
regcomp
().REG_LITERAL
REG_NOSPEC
.REG_ICASE
REG_NOSUB
REG_NEWLINE
[^
’
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
REG_PEND
is not recognized by any of
the wide character or “n
” variants.
Besides, the “n
” variants can be
used instead of REG_PEND
; see EXTENDED APIS
below.) 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.REG_ENHANCED
REG_MINIMAL
REG_ENHANCED
and
REG_EXTENDED
are also set.) This is an extension
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_UNGREEDY
REG_MINIMAL
.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.
The
regexec
()
function 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
^
’ anchor should not match before
it. This does not affect the behavior of newlines under
REG_NEWLINE
.REG_NOTEOL
$
’ anchor should not match before
it. This does not affect the behavior of newlines under
REG_NEWLINE
.REG_STARTEND
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.)
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
().
The
regerror
()
function 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. The (regerror
() may
be able to supply a more detailed message using information from the
regex_t.) The regerror
()
function 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 will not
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
(“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.
The
regfree
()
function 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.
These extended APIs are available in Mac OS X 10.8 and beyond,
when the deployment target is 10.8 or later. It should also be noted that
any of the
regcomp
()
variants may be used to initialize a regex_t
structure, that can then be passed to any of the
regexec
() variants. So it is quite legal to compile
a wide character RE and use it to match a multibyte character string, or
vice versa.
The
regncomp
()
routine compiles regular expressions like regcomp
(),
but the length of the regular expression string is specified, allowing a
string that is not NUL terminated and/or contains NUL characters. This is a
modern replacement for using regcomp
() with the
REG_PEND
option.
Similarly, the
regnexec
()
routine is like regexec
(), but the length of the
string to match is specified, allowing a string that is not NUL terminated
and/or contains NUL characters.
The
regwcomp
()
and
regwexec
()
variants take a wide-character (wchar_t) string for
the regular expression and string to match. And
regwncomp
()
and
regwnexec
()
are variants that allow specifying the wide character string length, and so
allows wide character strings that are not NUL terminated and/or contains
NUL characters.
When regcomp
() or one of its variants is
run, the regular expression is compiled into an internal form, which may
include specific information about the locale currently in effect, such as
equivalence classes or multi-character collation symbols. So a reference to
the current locale is also stored with the internal form, so that when
regexec
() is run, it can use the same locale (even
if the locale is changed in-between the calls to
regcomp
() and
regexec
()).
To provide more direct control over which locale is
used, routines with “_l
” appended to
their names are provided that work just like the variants without the
“_l
”, except that a locale (via a
locale_t variable type) is specified directly. Note
that only variants of
regcomp
()
have “_l
” variants, since the
regexec
() variants just use the reference to the
locale stored in the internal form.
The regex
implementation in Mac OS X 10.8
and later is based on a heavily modified subset of TRE
(http://laurikari.net/tre/). This provides improved performance, better
conformance and additional features. However, both API and binary
compatibility have been maintained with previous releases, so binaries built
on previous releases should work on 10.8 and later, and binaries built on
10.8 and later should be able to run on previous releases (as long as none
of the new variants or new features are used.
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 [“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, except for the use of
‘?
’ for minimal repetition (for
enhanced extended REs; see
re_format(7) for details). 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.
Non-zero error codes from regcomp
() and
regexec
() include the following:
REG_NOMATCH
regexec
() function failed to matchREG_BADPAT
REG_ECOLLATE
REG_ECTYPE
REG_EESCAPE
\
’ applied to unescapable
characterREG_ESUBREG
REG_EBRACK
[ ]
’ not balancedREG_EPAREN
( )
’ not balancedREG_EBRACE
{ }
’ not balancedREG_BADBR
{
}
’REG_ERANGE
[
]
’REG_ESPACE
REG_BADRPT
?
’,
‘*
’, or
‘+
’ operand invalidREG_EMPTY
REG_ASSERT
REG_INVARG
REG_ILLSEQ
IEEE Std 1003.2 (“POSIX.2”), sections 2.8 (Regular Expression Notation) and B.5 (C Binding for Regular Expression Matching).
The regex
implementation is based on a
heavily modified subset of TRE (http://laurikari.net/tre/), originally
written by Ville Laurikari. Previous releases used an implementation
originally written by Henry Spencer, and altered for
inclusion in the 4.4BSD distribution.
The beginning-of-line and end-of-line anchors ( “^”
and “$”) are currently implemented so that repetitions can not
be applied to them. The standards are unclear about whether this is legal,
but other regex
packages do support this case. It is
best to avoid this non-portable (and not really very useful) case.
The back-reference code is subtle and doubts linger about its correctness in complex cases.
The regexec
() variants use one of two
internal matching engines. The normal one is linear worst-case time in the
length of the text being searched, and quadratic worst-case time in the
length of the used regular expression. When back-references are used, a
slower, backtracking engine is used. While all backtracking matching engines
suffer from extreme slowness for certain pathological cases, the normal
engines doesn't suffer from these cases. It is advised to avoid
back-references whenever possible.
The regcomp
() variants 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.
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 cannot 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.
Sept 29, 2011 | macOS 14.6 |