regcomp

Syntax

#include <sys/types.h> #include <regex.h> int regcomp(regex_t *preg, const char *pattern, int cflags);

Description

This function is part of the implementation of POSIX 1003.2 regular expressions (REs).

regcomp compiles the regular expression contained in the pattern string, subject to the flags in cflags, and places the results in the regex_t structure pointed to by preg. (The regular expression syntax, as defined by POSIX 1003.2, is described below.)

The parameter cflags is the bitwise OR of zero or more of the following flags:

REG_EXTENDED

Compile modern (extended) REs, rather than the obsolete (basic) REs that are the default.

REG_BASIC

This is a synonym for 0, provided as a counterpart to REG_EXTENDED to improve readability. This is an extension, compatible with but not specified by POSIX 1003.2, and should be used with caution in software intended to be portable to other systems.

REG_NOSPEC

Compile with recognition of all special characters turned off. All characters are thus considered ordinary, so the RE in pattern is a literal string. This is an extension, compatible with but not specified by POSIX 1003.2, and should be used with caution in software intended to be portable to other systems. REG_EXTENDED and REG_NOSPEC may not be used in the same call to regcomp.

REG_ICASE

Compile for matching that ignores upper/lower case distinctions. See the description of regular expressions below for details of case-independent matching.

REG_NOSUB

Compile for matching that need only report success or failure, not what was matched.

REG_NEWLINE

Compile for newline-sensitive matching. By default, newline is a completely ordinary character with no special meaning in either REs or strings. With this flag, `[^' bracket expressions and `.' never match newline, a `^' anchor matches the null string after any newline in the string in addition to its normal function, and the `$' anchor matches the null string before any newline in the string in addition to its normal function.

REG_PEND

The regular expression ends, not at the first NUL, but just before the character pointed to by the re_endp member of the structure pointed to by preg. The re_endp member is of type const char *. This flag permits inclusion of NULs in the RE; they are considered ordinary characters. This is an extension, compatible with but not specified by POSIX 1003.2, and should be used with caution in software intended to be portable to other systems.

When successful, regcomp returns 0 and fills in the structure pointed to by preg. One member of that structure (other than re_endp) is publicized: re_nsub, of type size_t, contains the number of parenthesized subexpressions within the RE (except that the value of this member is undefined if the REG_NOSUB flag was used).

Note that the length of the RE does matter; in particular, there is a strong speed bonus for keeping RE length under about 30 characters, with most special characters counting roughly double.

Return Value

If regcomp succeeds, it returns zero; if it fails, it returns a non-zero error code, which is one of these:

REG_BADPAT

invalid regular expression

REG_ECOLLATE

invalid collating element

REG_ECTYPE

invalid character class

REG_EESCAPE

`\' applied to unescapable character

REG_ESUBREG

invalid backreference number (e.g., larger than the number of parenthesized subexpressions in the RE)

REG_EBRACK

brackets [ ] not balanced

REG_EPAREN

parentheses ( ) not balanced

REG_EBRACE

braces { } not balanced

REG_BADBR

invalid repetition count(s) in { }

REG_ERANGE

invalid character range in [ ]

REG_ESPACE

ran out of memory (an RE like, say, `((((a{1,100}){1,100}){1,100}){1,100}){1,100}'' will eventually run almost any existing machine out of swap space)

REG_BADRPT

?, *, or + operand invalid

REG_EMPTY

empty (sub)expression

REG_ASSERT

"can't happen" (you found a bug in regcomp)

REG_INVARG

invalid argument (e.g. a negative-length string)

Regular Expressions' Syntax

Regular expressions (REs), as defined in POSIX 1003.2, come in two forms: modern REs (roughly those of egrep; 1003.2 calls these extended REs) and obsolete REs (roughly those of ed; 1003.2 basic REs). Obsolete REs mostly exist for backward compatibility in some old programs; they will be discussed at the end. 1003.2 leaves some aspects of RE syntax and semantics open; `(*)' marks decisions on these aspects that may not be fully portable to other 1003.2 implementations.

A (modern) RE is one(*) or more non-empty(*) 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, etc.

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, e.g. `[0-9]' in ASCII matches any decimal digit. It is illegal(*) for two ranges to share an endpoint, e.g. `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 multi-character 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 multi-character collating element can thus match more than one character, e.g. 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 by isalnum (see section isalnum), isdigit (see section isdigit), ispunct (see section ispunct), isalpha (see section isalpha), isgraph (see section isgraph), isspace (see section isspace) (blank is the same as space), islower (see section islower), isupper (see section isupper), iscntrl (see section iscntrl), isprint (see section isprint), and isxdigit (see section isxdigit), respectively. A locale may provide others. A character class may not be used as an endpoint of a range.

There are two special cases(*) of bracket expressions: the bracket expressions `[[:<:]]' and `[[:>:]]' match the null string at the beginning and end of a word respectively. A word is defined as a sequence of word characters which is neither preceded nor followed by word characters. A word character is an alnum character (as defined by isalnum library function) or an underscore. This is an extension, compatible with but not specified by POSIX 1003.2, and should be used with caution in software intended to be portable to other systems.

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, e.g. `x' becomes `[xX]'. When it appears inside a bracket expression, all case counterparts of it are added to the bracket expression, so that (e.g.) `[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 non-zero 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 (e.g.) `\([bc]\)\1' matches "bb" or "cc" but not "bc".

History

This implementation of the POSIX regexp functionality was written by Henry Spencer.

Bugs

The locale is always assumed to be the default one of 1003.2, and only the collating elements etc. of that locale are available.

regcomp implements bounded repetitions by macro expansion, which is costly in time and space if counts are large or bounded repetitions are nested.

An RE like, say, `((((a{1,100}){1,100}){1,100}){1,100}){1,100}', will (eventually) run almost any existing machine out of swap space.

There are suspected problems with response to obscure error conditions. Notably, certain kinds of internal overflow, produced only by truly enormous REs or by multiply nested bounded repetitions, are probably not handled well.

Due to a mistake in 1003.2, things like `a)b' are legal REs because `)' is a special character only in the presence of a previous unmatched `('. This can't be fixed until the spec is fixed.

The standard's definition of back references is vague. For example, does `a\e(\e(b\e)*\e2\e)*d' match "abbbd"? Until the standard is clarified, behavior in such cases should not be relied on.

Portability

ANSI/ISO C	No
POSIX	1003.2-1992; 1003.1-2001