Ubuntu Manpage: regcomp, regexec, regerror, regfree

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 zero 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
Do not report position of matches. The nmatch and pmatch arguments 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 nonmatching 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 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 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).

REG_STARTEND
Use pmatch[0] on the input string, starting at byte pmatch[0].rm_so and ending before byte
pmatch[0].rm_eo. This allows matching embedded NUL bytes and avoids a strlen(3) on large strings.
It does not use nmatch on input, and does not change REG_NOTBOL or REG_NEWLINE processing. This
flag is a BSD extension, not present in POSIX.

Byte offsets
Unless REG_NOSUB was set for the compilation of the pattern buffer, it is possible to obtain match
addressing information. pmatch must be dimensioned to have at least nmatch elements. These are filled
in by regexec() with substring match addresses. The offsets of the subexpression starting at the ith
open parenthesis are stored in pmatch[i]. The entire regular expression's match addresses are stored in
pmatch[0]. (Note that to return the offsets of N subexpression matches, nmatch must be at least N+1.)
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, which is the offset of the
first character after the matching text.

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
nonzero, errbuf is filled in with the first errbuf_size - 1 characters of the error message and a
terminating null byte ('\0').

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().

RETURN VALUE

       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.

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
              Nonspecific 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; for example, the ending point of the range occurs prior to the
              starting point.

       REG_ESIZE
              Compiled regular expression requires a pattern buffer larger than 64 kB.  This is not  defined  by
              POSIX.2.

       REG_ESPACE
              The regex routines ran out of memory.

       REG_ESUBREG
              Invalid back reference to a subexpression.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌─────────────────────┬───────────────┬────────────────┐
       │Interface            │ Attribute     │ Value          │
       ├─────────────────────┼───────────────┼────────────────┤
       │regcomp(), regexec() │ Thread safety │ MT-Safe locale │
       ├─────────────────────┼───────────────┼────────────────┤
       │regerror()           │ Thread safety │ MT-Safe env    │
       ├─────────────────────┼───────────────┼────────────────┤
       │regfree()            │ Thread safety │ MT-Safe        │
       └─────────────────────┴───────────────┴────────────────┘

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008.

COLOPHON

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