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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
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() 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
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 REG_NOTBOL.
Match-end-of-line operator ($) matches the empty string
immediately before a newline, regardless of whether eflags
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 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 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
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 match.
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 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 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() returns zero for a successful compilation or an error code
regexec() returns zero for a successful match or REG_NOMATCH for
The following errors can be returned by regcomp():
Invalid use of back reference operator.
Invalid use of pattern operators such as group or list.
Invalid use of repetition operators such as using ‘*’ as the
Un-matched brace interval operators.
Un-matched bracket list operators.
Invalid collating element.
Unknown character class name.
Non specific error. This is not defined by POSIX.2.
Un-matched parenthesis group operators.
Invalid use of the range operator, eg. the ending point of the
range occurs prior to the starting point.
Compiled regular expression requires a pattern buffer larger
than 64Kb. This is not defined by POSIX.2.
The regex routines ran out of memory.
Invalid back reference to a subexpression.
regex(7), GNU regex manual