REGCOMP(3) Linux Programmer's Manual REGCOMP(3)
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,
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 subse-
quent 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 com-
All regular expression searching must be done via a compiled pattern buffer, thus regexec
must always be supplied with the address of a regcomp initialized pattern buffer.
cflags may be the bitwise-or of one or more of the following:
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, contains
Match-end-of-line operator ($) matches the empty string immediately before a new-
line, 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 line.
The match-end-of-line operator always fails to match (but see the compilation flag
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 substring 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 strings.
regerror is passed the error code, errcode, the pattern buffer, preg, a pointer to a char-
acter 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 terminating null.
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 for failure.
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.
regex(7), GNU regex manual
GNU 1998-05-08 REGCOMP(3)