Manpage of REGEX

REGEX

Section: Linux Programmer's Manual (3)
Updated: 2015-08-08
Index
 

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.

cflagsmay be the bitwise-orof zero or more of the following:

REG_EXTENDED
Use POSIXExtended Regular Expression syntax when interpreting regex. If not set, POSIXBasic 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 nmatchand pmatcharguments 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 eflagscontains REG_NOTEOL.

 

POSIX regex matching

regexec() is used to match a null-terminated string against the precompiled pattern buffer, preg. nmatchand pmatchare used to provide information regarding the location of any matches. eflagsmay be the bitwise-orof one or both of REG_NOTBOLand REG_NOTEOLwhich 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_NEWLINEabove). 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_NEWLINEabove).
 

Byte offsets

Unless REG_NOSUBwas set for the compilation of the pattern buffer, it is possible to obtain match addressing information. pmatchmust be dimensioned to have at least nmatchelements. 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 Nsubexpression matches, nmatchmust be at least N+1.) Any unused structure elements will contain the value -1.

The regmatch_tstructure which is the type of pmatchis defined in <regex.h>.

typedef struct {
    regoff_t rm_so;
    regoff_t rm_eo;
} regmatch_t;

Each rm_soelement that is not -1 indicates the start offset of the next largest substring match within the string. The relative rm_eoelement 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 errbufrequired to contain the null-terminated error message string. If both errbufand errbuf_sizeare nonzero, errbufis filled in with the first errbuf_size - 1characters of the error message and a terminating null byte (aq\0aq).  

POSIX pattern buffer freeing

Supplying regfree() with a precompiled pattern buffer, pregwill 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_NOMATCHfor 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 aq*aq 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 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.
 

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).
InterfaceAttributeValue
regcomp(), regexec() Thread safetyMT-Safe locale
regerror() Thread safetyMT-Safe env
regfree() Thread safetyMT-Safe
 

CONFORMING TO

POSIX.1-2001, POSIX.1-2008.  

SEE ALSO

grep(1), regex(7)
The glibc manual section, Regular Expression Matching


 

Index

NAME
SYNOPSIS
DESCRIPTION
POSIX regex compiling
POSIX regex matching
Byte offsets
POSIX error reporting
POSIX pattern buffer freeing
RETURN VALUE
ERRORS
ATTRIBUTES
CONFORMING TO
SEE ALSO

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Time: 22:28:01 GMT, June 20, 2016