E.3. Regular expressions

Regular expressions are described in the manual pages for the regex functions provided with the system libraries. The following is extracted from the regex(7) (or re_format(7) on FreeBSD) manual pages:

A regular expression (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 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 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 "x" and "y" are the members of an equivalence class, then "[[=x=]]", "[[=y=]]", and "[xy]" 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 and xdigit.

These stand for the character classes defined in ctype(3). 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 ctype(3)) or an underscore. This is an extension, compatible with but not specified by IEEE Std 1003.2 ("POSIX.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]".