REGEX(3) | Library Functions Manual | REGEX(3) |
regex
, 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[], 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
() 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
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
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.
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.
regcomp
() and
regexec
() include the following:
REG_NOMATCH
regexec
() failed to matchREG_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 regnsub
() and
regasub
() functions appeared in
NetBSD 8.
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.
February 26, 2018 | NetBSD 9.2 |