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NAME
regcomp, regexec, regerror, regfree - POSIX regex functions
SYNOPSIS
#include <sys/types.h>
#include <regex.h>
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);
DESCRIPTION
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:
REG_EXTENDED
Use POSIX Extended Regular Expression syntax when interpreting
regex. If not set, POSIX Basic Regular Expression syntax is
used.
REG_ICASE
Do not differentiate case. Subsequent regexec() searches using
this pattern buffer will be case insensitive.
REG_NOSUB
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.
REG_NEWLINE
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 immedi‐
ately before a newline, regardless of whether eflags contains
REG_NOTEOL.
POSIX Regex Matching
regexec() is used to match a null-terminated string against the precom‐
piled 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 behavior described below.
REG_NOTBOL
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.
REG_NOTEOL
The match-end-of-line operator always fails to match (but see
the compilation flag REG_NEWLINE above)
BYTE OFFSETS
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.
typedef struct {
regoff_t rm_so;
regoff_t rm_eo;
} regmatch_t;
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().
regcomp() returns zero for a successful compilation or an error code
for failure.
regexec() returns zero for a successful match or REG_NOMATCH for fail‐
ure.
ERRORS
The following errors can be returned by regcomp():
REG_BADBR
Invalid use of back reference operator.
REG_BADPAT
Invalid use of pattern operators such as group or list.
REG_BADRPT
Invalid use of repetition operators such as using ‘*’ as the
first character.
REG_EBRACE
Un-matched brace interval operators.
REG_EBRACK
Un-matched bracket list operators.
REG_ECOLLATE
Invalid collating element.
REG_ECTYPE
Unknown character class name.
REG_EEND
Non specific error. This is not defined by POSIX.2.
REG_EESCAPE
Trailing backslash.
REG_EPAREN
Un-matched parenthesis group operators.
REG_ERANGE
Invalid use of the range operator, eg. the ending point of the
range occurs prior to the starting point.
REG_ESIZE
Compiled regular expression requires a pattern buffer larger
than 64Kb. This is not defined by POSIX.2.
REG_ESPACE
The regex routines ran out of memory.
REG_ESUBREG
Invalid back reference to a subexpression.
POSIX.1-2001.
grep(1), regex(7), GNU regex manual