REGEX(3) | Library Functions Manual | REGEX(3) |
regcomp
, regexec
,
regerror
, regfree
,
regasub
, regnsub
—
#include <regex.h>
int
regcomp
(regex_t * restrict preg,
const char * restrict pattern, int
cflags);
int
regexec
(const regex_t * restrict
preg, const char * restrict string,
size_t nmatch, regmatch_t
pmatch[restrict], int eflags);
size_t
regerror
(int errcode,
const regex_t * restrict preg, char *
restrict errbuf, size_t errbuf_size);
void
regfree
(regex_t
*preg);
ssize_t
regnsub
(char
*buf, size_t
bufsiz, const char
*sub, const regmatch_t
*rm, const char
*str);
ssize_t
regasub
(char
**buf, const char
*sub, const regmatch_t
*rm, const char
*sstr);
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, 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_
”.
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_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_GNU
N
where
N
is a single digit number between
1
and 9
.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.
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
^
’,
‘[[:<:]]
’, and
‘\<
’ do not match before it; but
see REG_STARTEND
below. This does not affect the
behavior of newlines under REG_NEWLINE
.REG_NOTEOL
$
’ anchor does not match before it.
This does not affect the behavior of newlines under
REG_NEWLINE
.REG_STARTEND
Without REG_NOTBOL
, the position
rm_so is considered the beginning of a line, such
that ‘^
’ matches before it, and
the beginning of a word if there is a word character at this position,
such that ‘[[:<:]]
’ and
‘\<
’ match before it.
With REG_NOTBOL
, the character at
position rm_so is treated as the continuation of a
line, and if rm_so is greater than 0, the
preceding character is taken into consideration. If the preceding
character is a newline and the regular expression was compiled with
REG_NEWLINE
,
‘^
’ matches before the string; if
the preceding character is not a word character but the string starts
with a word character,
‘[[:<:]]
’ and
‘\<
’ match before the
string.
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.
The regnsub
() and
regasub
() functions perform substitutions using
sed(1) like syntax. They return
the length of the string that would have been created if there was enough
space or -1
on error, setting
errno
. The result is being placed in
buf which is user-supplied in
regnsub
() and dynamically allocated in
regasub
(). The sub argument
contains a substitution string which might refer to the first 9 regular
expression strings using “\<n>” to refer to the nth
matched item, or “&” (which is equivalent to
“\0”) to refer to the full match. The rm
array must be at least 10 elements long, and should contain the result of
the matches from a previous regexec
() call. Only 10
elements of the rm array can be used. The
str argument contains the source string to apply the
transformation to.
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. 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.
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 regnsub
() and
regasub
() functions appeared in
NetBSD 8.
The back-reference code is subtle and doubts linger about its correctness in complex cases.
The regexec
() function performance is
poor. This will improve with later releases. The
nmatch argument exceeding 0 is expensive;
nmatch exceeding 1 is worse. The
regexec
() function 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.
The regcomp
() function 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 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.
The implementation of word-boundary matching is a bit of a kludge, and bugs may lurk in combinations of word-boundary matching and anchoring.
Word-boundary matching does not work properly in multibyte locales.
March 11, 2021 | NetBSD 10.0 |