DESCRIPTION
Regular expressions ("REs"), as defined in -p1003.2, come in two forms: modern REs (roughly those of egrep(1); 1003.2 calls these "extended" REs) and obsolete REs (roughly those of ed(1); 1003.2 "basic" REs). Obsolete REs mostly exist for backward compatibility in some old programs; they will be discussed at the end. -p1003.2 leaves some aspects of RE syntax and semantics open; ‘‡’ marks decisions on these aspects that may not be fully portable to other -p1003.2 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 ‘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:
