NAME
regex
, regcomp
,
regexec
, regerror
,
regfree
, regasub
,
regnsub
—
regular-expression library
LIBRARY
library “libc”
SYNOPSIS
#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[], 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);
DESCRIPTION
These routines implement IEEE Std 1003.2-1992 (“POSIX.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
IEEE Std 1003.2-1992 (“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 `$' anchor matches the null string before any newline in the string in addition to its normal function.
REG_PEND
- The regular expression ends, not at the first NUL, but just before the character pointed to by the re_endp member of the structure pointed to by preg. The re_endp member is of type const char *. This flag permits inclusion of NULs in the RE; they are considered ordinary characters. This is an extension, compatible with but not specified by IEEE Std 1003.2-1992 (“POSIX.2”), and should be used with caution in software intended to be portable to other systems.
When successful,
regcomp
()
returns 0 and fills in the structure pointed to by
preg. One member of that structure (other than
re_endp) is publicized: re_nsub,
of type size_t, contains the number of parenthesized
subexpressions within the RE (except that the value of this member is
undefined if the REG_NOSUB
flag was used). If
regcomp
() fails, it returns a non-zero error code;
see DIAGNOSTICS.
regexec
()
matches the compiled RE pointed to by preg against the
string, subject to the flags in
eflags, and reports results using
nmatch, pmatch, and the returned
value. The RE must have been compiled by a previous invocation of
regcomp
(). The compiled form is not altered during
execution of regexec
(), so a single compiled RE can
be used simultaneously by multiple threads.
By default, the NUL-terminated string pointed to by string is considered to be the text of an entire line, minus any terminating newline. The eflags argument is the bitwise OR of zero or more of the following flags:
REG_NOTBOL
- The first character of the string is not the beginning of a line, so the
`^' anchor should not match before it. This does not affect the behavior
of newlines under
REG_NEWLINE
. REG_NOTEOL
- The NUL terminating the string does not end a line, so the `$' 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 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.
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.
IMPLEMENTATION CHOICES
There are a number of decisions that IEEE Std 1003.2-1992 (“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 1003.2-1992 (“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 `{' 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 matchREG_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 1003.2-1992 (“POSIX.2”), sections 2.8 (Regular Expression Notation) and B.5 (C Binding for Regular Expression Matching).
HISTORY
Originally written by Henry Spencer. Altered for inclusion in the 4.4BSD distribution.
The regnsub
() and
regasub
() functions appeared in
NetBSD 8.
BUGS
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 1003.2-1992 (“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. 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.