regcomp, regexec, regerror, regfree - POSIX regex functions
int regcomp(regex_t *preg, const char *regex, 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));
POSIX REGEX COMPILING
regcomp is used to compile a regular expression into a form
that is suitable for subsequent regexec searches.
regcomp is supplied with preg, a pointer to a pattern buffer
storage area; regex, a pointer to the null-terminated string
and cflags, flags used to determine the type of compilation.
All regular expression searching must be done via a compiled
pattern buffer, thus regexec must always be supplied with
the address of a regcomp initialised pattern buffer.
cflags may be the bitwise-or of one or more of the follow-
Use POSIX Extended Regular Expression syntax when
interpreting regex. If not set, POSIX Basic Regular
Expression syntax is used.
Do not differentiate case. Subsequent regexec searches
using this pattern buffer will be case insensitive.
Support for substring addressing of matches is not
required. The nmatch and pmatch parameters to regexec
are ignored if the pattern buffer supplied was compiled
with this flag set.
Match-any-character operators don't match a newline.
A non-matching list ([^...]) not containing a newline
does not match a newline.
Match-beginning-of-line operator (^) matches the empty
string immediately after a newline, regardless of
whether eflags, the execution flags of regexec,
Match-end-of-line operator ($) matches the empty string
immediately before a newline, regardless of whether
eflags contains REG_NOTEOL.
POSIX REGEX MATCHING
regexec is used to match a null-terminated string against
the precompiled pattern buffer, preg. nmatch and pmatch are
used to provide information regarding the location of any
matches. eflags may be the bitwise-or of one or both of
REG_NOTBOL and REG_NOTEOL which cause changes in matching
behaviour described below.
The match-beginning-of-line operator always fails to
match (but see the compilation flag REG_NEWLINE above)
This flag may be used when different portions of a
string are passed to regexec and the beginning of the
string should not be interpreted as the beginning of
The match-end-of-line operator always fails to match
(but see the compilation flag REG_NEWLINE above)
Unless REG_NOSUB was set for the compilation of the pattern
buffer, it is possible to obtain substring match addressing
information. pmatch must be dimensioned to have at least
nmatch elements. These are filled in by regexec with sub-
string match addresses. Any unused structure elements will
contain the value -1.
The regmatch_t structure which is the type of pmatch is
defined in regex.h.
Each rm_so element that is not -1 indicates the start offset
of the next largest substring match within the string. The
relative rm_eo element indicates the end offset of the
POSIX ERROR REPORTING
regerror is used to turn the error codes that can be
returned by both regcomp and regexec into error message
regerror is passed the error code, errcode, the pattern
buffer, preg, a pointer to a character string buffer,
errbuf, and the size of the string buffer, errbuf_size. It
returns the size of the errbuf required to contain the
null-terminated error message string. If both errbuf and
errbuf_size are non-zero, errbuf is filled in with the first
errbuf_size - 1 characters of the error message and a ter-
POSIX PATTERN BUFFER FREEING
Supplying regfree with a precompiled pattern buffer, preg
will free the memory allocated to the pattern buffer by the
compiling process, regcomp.
regcomp returns zero for a successful compilation or an
error code for failure.
regexec returns zero for a successful match or REG_NOMATCH
The following errors can be returned by regcomp:
Invalid use of repetition operators such as using `*'
as the first character.
Invalid use of back reference operator.
Un-matched brace interval operators.
Un-matched bracket list operators.
Invalid use of the range operator, eg. the ending point
of the range occurs prior to the starting point.
Unknown character class name.
Invalid collating element.
Un-matched parenthesis group operators.
Invalid back reference to a subexpression.
Non specific error. This is not defined by POSIX.2.
Invalid use of pattern operators such as group or list.
Compiled regular expression requires a pattern buffer
larger than 64Kb. This is not defined by POSIX.2.
The regex routines ran out of memory.
Currently (GNU libc snapshot 980503), GNU libc does not sup-
port collating elements in regular expressions.
regex(7), GNU regex manual