| REGEXEC(3) | Library Functions Manual | REGEXEC(3) |
regcomp, regexec,
regerror, regfree —
regular expression routines
#include
<sys/types.h>
#include <regex.h>
int
regcomp(regex_t
*preg, const char
*pattern, int
cflags);
int
regexec(const
regex_t *preg, const char
*string, size_t
nmatch, regmatch_t
pmatch[], int
eflags);
size_t
regerror(int
errcode, const regex_t
*preg, char
*errbuf, size_t
errbuf_size);
void
regfree(regex_t
*preg);
These routines implement IEEE Std 1003.2
(“POSIX.2”) regular expressions (“REs”);
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. The cflags argument is the
bitwise OR of zero or more of the following values:
REG_EXTENDEDREG_BASICREG_EXTENDED to improve readability.REG_NOSPECREG_EXTENDED and
REG_NOSPEC may not be used in the same call to
regcomp().REG_ICASEREG_NOSUBREG_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 ‘$’ anchor matches
the null string before any newline in the string in addition to its normal
function.REG_PENDWhen successful,
regcomp()
returns 0 and fills in the structure pointed to by
preg. One member of that structure (other than
re_endp) is publicized: re_nsub,
of type size_t, contains the number of parenthesized
subexpressions within the RE (except that the value of this member is
undefined if the REG_NOSUB flag was used). If
regcomp() fails, it returns a non-zero error code;
see DIAGNOSTICS.
regexec()
matches the compiled RE pointed to by preg against the
string, subject to the flags in
eflags, and reports results using
nmatch, pmatch, and the returned
value. The RE must have been compiled by a previous invocation of
regcomp(). The compiled form is not altered during
execution of regexec(), so a single compiled RE can
be used simultaneously by multiple threads.
By default, the NUL-terminated string pointed to by string is considered to be the text of an entire line, minus any terminating newline. The eflags argument is the bitwise OR of zero or more of the following values:
REG_NOTBOL^’,
‘[[:<:]]’, and
‘\<’ do not match before it; but
see REG_STARTEND below. This does not affect the
behavior of newlines under REG_NEWLINE.REG_NOTEOL$’ anchor does not match before it.
This does not affect the behavior of newlines under
REG_NEWLINE.REG_STARTENDWithout 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 ‘bs’ 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 the 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 OR'ed 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.
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.
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 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
‘|’.
A ‘|’ 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.
Non-zero error codes from regcomp() and
regexec() include the following:
REG_NOMATCHREG_BADPATREG_ECOLLATEREG_ECTYPEREG_EESCAPEREG_ESUBREGREG_EBRACKREG_EPARENREG_EBRACEREG_BADBRREG_ERANGEREG_ESPACEREG_BADRPTREG_EMPTYREG_ASSERTREG_INVARGIEEE Std 1003.2 (“POSIX.2”), sections 2.8 (Regular Expression Notation) and B.5 (C Binding for Regular Expression Matching).
Predecessors to regcomp() and
regexec(), regcmp() and
regex(), first appeared in the Programmer's
Workbench (PWB/UNIX).
The present functions were originally written by Henry Spencer and altered for inclusion in the 4.4BSD distribution.
This is an alpha release with known defects. Please report problems.
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 (“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. A 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 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.
| May 26, 2016 | OpenBSD-6.7 |