1. (regexp, RE) One of the wild card patterns used by Unix utilities such as grep, sed and awk and editors such as vi and Emacs. These use conventions similar to but more elaborate than those described under glob. A regular expression is a sequence of characters with the following meanings:

An ordinary character (not one of the special characters discussed below) matches that character.

A backslash (\) followed by any special character matches the special character itself. The special characters are:

"." matches any character except NEWLINE; "RE*" (where the "*" is called the "Kleene star") matches zero or more occurrences of RE. If there is any choice, the longest leftmost matching string is chosen, in most regexp flavours.

"^" at the beginning of an RE matches the start of a line and "$" at the end of an RE matches the end of a line.

[string] matches any one character in that string. If the first character of the string is a "^" it matches any character (except NEWLINE, in most regexp flavours) and the remaining characters in the string. "-" may be used to indicate a range of consecutive ASCII characters.

\( RE \) matches whatever RE matches and \n, where n is a digit, matches whatever was matched by the RE between the nth \( and its corresponding \) earlier in the same RE. In many flavours ( RE ) is used instead of \( RE \)

The concatenation of REs is a RE that matches the concatenation of the strings matched by each RE.

\matches the end of a word. In many flavours of regexp, \> and \< are replaced by "\b", the special character for "word boundary".

RE[m] matches m occurences of RE. RE[m,] matches m or more occurences of RE. RE[m,n] matches between m and n occurences.

The exact details of how regexp will work in a given application vary greatly from flavour to flavour. A comprehensive survey of regexp flavours is found in Friedl 1997 (see below).

[Jeffrey E.F. Friedl, "Mastering Regular Expressions, O'Reilly, 1997.]

2. Any description of a pattern composed from combinations of symbols and the three operators:

Concatenation - pattern A concatenated with B matches a match for A followed by a match for B.

Or - pattern A-or-B matches either a match for A or a match for B.

Closure - zero or more matches for a pattern.

The earliest form of regular expressions (and the term itself) were invented by mathematician Stephen Cole Kleene in the mid-1950s, as a notation to easily manipulate "regular sets", formal descriptions of the behaviour of finite state machines, in regular algebra.

[S.C. Kleene, "Representation of events in nerve nets and finite automata", 1956, Automata Studies. Princeton].

[J.H. Conway, "Regular algebra and finite machines", 1971, Eds Chapman & Hall].

[Sedgewick, "Algorithms in C", page 294].