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REGEX(3) Library Functions Manual REGEX(3)
NAME
regcomp, regexec, regerror, regfree, regasub, regnsub - regular-
expression library
LIBRARY
Standard C Library (libc, -lc)
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[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);
DESCRIPTION
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, 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 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 and REG_NOSPEC may
not be used in the same call to regcomp().
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.
REG_GNU Include GNU-inspired extensions:
\N Use backreference N where N is a single digit
number between 1 and 9.
\a Visual Bell
\b Match a position that is a word boundary.
\B Match a position that is a not word boundary.
\f Form Feed
\n Line Feed
\r Carriage return
\s Alias for [[:space:]]
\S Alias for [^[:space:]]
\t Horizontal Tab
\v Vertical Tab
\w Alias for [[:alnum:]_]
\W Alias for [^[:alnum:]_]
\' Matches the end of the subject string (the string
to be matched).
\` Matches the beginning of the subject 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.
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 The first character of the string is treated as the
continuation of a line. This means that the anchors `^',
`[[:<:]]', and `\<' do not match before it; but see
REG_STARTEND below. 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 does 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 end before the byte located at
string + pmatch[0].rm_eo, 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.
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.
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 ["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.
RETURN VALUES
Non-zero error codes from regcomp() and regexec() include the following:
REG_NOMATCH The regexec() function 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 cannot happen - you found a bug
REG_INVARG invalid argument, e.g. negative-length string
REG_ILLSEQ illegal byte sequence (bad multibyte character)
SEE ALSO
grep(1), re_format(7)
IEEE Std 1003.2 ("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
This is an alpha release with known defects. Please report problems.
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.
NetBSD 10.99 March 11, 2021 NetBSD 10.99