[MAN] regex

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REGEX

Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
Index Return to Main Contents

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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 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).

 

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.  

SEE ALSO

grep(1), regex(7)

The glibc manual section, Regular Expressions  

COLOPHON

This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

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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

COLOPHON

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REGEX

Section: Linux Programmer's Manual (7)
Updated: 2009-01-12
Index Return to Main Contents

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NAME

regex - POSIX.2 regular expressions  

DESCRIPTION

Regular expressions ("RE"s), as defined in POSIX.2, come in two forms: modern REs (roughly those of egrep; POSIX.2 calls these "extended" REs) and obsolete REs (roughly those of ed(1); POSIX.2 "basic" REs). Obsolete REs mostly exist for backward compatibility in some old programs; they will be discussed at the end. POSIX.2 leaves some aspects of RE syntax and semantics open; "(!)" marks decisions on these aspects that may not be fully portable to other POSIX.2 implementations.

A (modern) RE is one(!) or more nonempty(!) branches, separated by '|'. It matches anything that matches one of the branches.

A branch is one(!) or more pieces, concatenated. It matches a match for the first, followed by a match for the second, and so on.

A piece is an atom possibly followed by a single(!) '*', '+', '?', or bound. An atom followed by '*' matches a sequence of 0 or more matches of the atom. An atom followed by '+' matches a sequence of 1 or more matches of the atom. An atom followed by '?' matches a sequence of 0 or 1 matches of the atom.

A bound is '{' followed by an unsigned decimal integer, possibly followed by ',' possibly followed by another unsigned decimal integer, always followed by '}'. The integers must lie between 0 and RE_DUP_MAX (255(!)) inclusive, and if there are two of them, the first may not exceed the second. An atom followed by a bound containing one integer i and no comma matches a sequence of exactly i matches of the atom. An atom followed by a bound containing one integer i and a comma matches a sequence of i or more matches of the atom. An atom followed by a bound containing two integers i and j matches a sequence of i through j (inclusive) matches of the atom.

An atom is a regular expression enclosed in "()" (matching a match for the regular expression), an empty set of "()" (matching the null string)(!), a bracket expression (see below), '.' (matching any single character), '^' (matching the null string at the beginning of a line), '$' (matching the null string at the end of a line), a '\' followed by one of the characters "^.[$()|*+?{\" (matching that character taken as an ordinary character), a '\' followed by any other character(!) (matching that character taken as an ordinary character, as if the '\' had not been present(!)), or a single character with no other significance (matching that character). A '{' followed by a character other than a digit is an ordinary character, not the beginning of a bound(!). It is illegal to end an RE with '\'.

A bracket expression is a list of characters enclosed in "[]". It normally matches any single character from the list (but see below). If the list begins with '^', it matches any single character (but see below) not from the rest of the list. If two characters in the list are separated by '-', this is shorthand for the full range of characters between those two (inclusive) in the collating sequence, for example, "[0-9]" in ASCII matches any decimal digit. It is illegal(!) for two ranges to share an endpoint, for example, "a-c-e". Ranges are very collating-sequence-dependent, and portable programs should avoid relying on them.

To include a literal ']' in the list, make it the first character (following a possible '^'). To include a literal '-', make it the first or last character, or the second endpoint of a range. To use a literal '-' as the first endpoint of a range, enclose it in "[." and ".]" to make it a collating element (see below). With the exception of these and some combinations using '[' (see next paragraphs), all other special characters, including '\', lose their special significance within a bracket expression.

Within a bracket expression, a collating element (a character, a multicharacter sequence that collates as if it were a single character, or a collating-sequence name for either) enclosed in "[." and ".]" stands for the sequence of characters of that collating element. The sequence is a single element of the bracket expression's list. A bracket expression containing a multicharacter collating element can thus match more than one character, for example, if the collating sequence includes a "ch" collating element, then the RE "[[.ch.]]*c" matches the first five characters of "chchcc".

Within a bracket expression, a collating element enclosed in "[=" and "=]" is an equivalence class, standing for the sequences of characters of all collating elements equivalent to that one, including itself. (If there are no other equivalent collating elements, the treatment is as if the enclosing delimiters were "[." and ".]".) For example, if o and are the members of an equivalence class, then "[[=o=]]", "[[==]]", and "[o]" are all synonymous. An equivalence class may not(!) be an endpoint of a range.

Within a bracket expression, the name of a character class enclosed in "[:" and ":]" stands for the list of all characters belonging to that class. Standard character class names are:

alnum	digit	punct
alpha	graph	space
blank	lower	upper
cntrl	print	xdigit

These stand for the character classes defined in wctype(3). A locale may provide others. A character class may not be used as an endpoint of a range.

In the event that an RE could match more than one substring of a given string, the RE matches the one starting earliest in the string. If the RE could match more than one substring starting at that point, it matches the longest. Subexpressions also match the longest possible substrings, subject to the constraint that the whole match be as long as possible, with subexpressions starting earlier in the RE taking priority over ones starting later. Note that higher-level subexpressions thus take priority over their lower-level component subexpressions.

Match lengths are measured in characters, not collating elements. A null string is considered longer than no match at all. For example, "bb*" matches the three middle characters of "abbbc", "(wee|week)(knights|nights)" matches all ten characters of "weeknights", when "(.*).*" is matched against "abc" the parenthesized subexpression matches all three characters, and when "(a*)*" is matched against "bc" both the whole RE and the parenthesized subexpression match the null string.

If case-independent matching is specified, the effect is much as if all case distinctions had vanished from the alphabet. When an alphabetic that exists in multiple cases appears as an ordinary character outside a bracket expression, it is effectively transformed into a bracket expression containing both cases, for example, 'x' becomes "[xX]". When it appears inside a bracket expression, all case counterparts of it are added to the bracket expression, so that, for example, "[x]" becomes "[xX]" and "[^x]" becomes "[^xX]".

No particular limit is imposed on the length of REs(!). Programs intended to be portable should not employ REs longer than 256 bytes, as an implementation can refuse to accept such REs and remain POSIX-compliant.

Obsolete ("basic") regular expressions differ in several respects. '|', '+', and '?' are ordinary characters and there is no equivalent for their functionality. The delimiters for bounds are "\{" and "\}", with '{' and '}' by themselves ordinary characters. The parentheses for nested subexpressions are "\(" and "\)", with '(' and ')' by themselves ordinary characters. '^' is an ordinary character except at the beginning of the RE or(!) the beginning of a parenthesized subexpression, '$' is an ordinary character except at the end of the RE or(!) the end of a parenthesized subexpression, and '*' is an ordinary character if it appears at the beginning of the RE or the beginning of a parenthesized subexpression (after a possible leading '^').

Finally, there is one new type of atom, a back reference: '\' followed by a nonzero decimal digit d matches the same sequence of characters matched by the dth parenthesized subexpression (numbering subexpressions by the positions of their opening parentheses, left to right), so that, for example, "\([bc]\)\1" matches "bb" or "cc" but not "bc".  

BUGS

Having two kinds of REs is a botch.

The current POSIX.2 spec says that ')' is an ordinary character in the absence of an unmatched '('; this was an unintentional result of a wording error, and change is likely. Avoid relying on it.

Back references are a dreadful botch, posing major problems for efficient implementations. They are also somewhat vaguely defined (does "a\(\(b\)*\2\)*d" match "abbbd"?). Avoid using them.

POSIX.2's specification of case-independent matching is vague. The "one case implies all cases" definition given above is current consensus among implementors as to the right interpretation.  

AUTHOR

This page was taken from Henry Spencer's regex package.  

SEE ALSO

grep(1), regex(3)

POSIX.2, section 2.8 (Regular Expression Notation).  

COLOPHON

This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

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Index

NAME

DESCRIPTION

BUGS

AUTHOR

SEE ALSO

COLOPHON

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This document was created by man2html, using the manual pages.
Time: 04:45:57 GMT, September 16, 2022