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[]
, int eflags
)
size_t
regerror(
int errcode
, const regex_t * restrict preg
, char * restrict errbuf
, size_t errbuf_size
)
void
regfree(
regex_t *preg
)
)
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
REG_BASIC
REG_NOSPEC
REG_EXTENDED
and
REG_NOSPEC
may not be used in the same call to
regcomp(
).
REG_ICASE
REG_NOSUB
REG_NEWLINE
REG_PEND
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 1003.2-1992 (``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
REG_NEWLINE
.
REG_NOTEOL
REG_NEWLINE
.
REG_STARTEND
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 1003.2-1992 (``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 `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().
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
IEEE Std 1003.2-1992 (``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.
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 1003.2-1992 (``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 `{' _n_o_t followed by a digit is considered an ordinary character.
`^' and `$' beginning and ending subexpressions in obsolete (``basic'') REs are anchors, not ordinary characters.
)
and
regexec(
)
include the following:
REG_NOMATCH
REG_BADPAT
REG_ECOLLATE
REG_ECTYPE
REG_EESCAPE
REG_ESUBREG
REG_EBRACK
REG_EPAREN
REG_EBRACE
REG_BADBR
REG_ERANGE
REG_ESPACE
REG_BADRPT
REG_EMPTY
REG_ASSERT
REG_INVARG
IEEE Std 1003.2-1992 (``POSIX.2'') , sections 2.8 (Regular Expression Notation) and B.5 (C Binding for Regular Expression Matching).
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 IEEE Std 1003.2-1992 (``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.