Adapted from Altap Salamander's Help. (c) 2001 ALTAP, Ltd. All rights reserved.
A regular expression is zero or more branches, separated by '|'. It matches anything that matches one of the branches.
A branch is zero 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 '*', '+', or '?'. 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 match of the atom, or the null string.
An atom is a regular expression in parentheses (matching a match for the regular expression), a range (see below), '.' (matching any single character), '^' (matching the null string at the beginning of line), '$' (matching the null string at the end of line), a '\' followed by a single character (matching that character), or a single character with no other significance (matching that character).
A range is a sequence of characters enclosed in ''. It normally matches any single character from the sequence. If the sequence begins with '^', it matches any single character not from the rest of the sequence. If two characters in the sequence are separated by '-', this is shorthand for the full list of ASCII characters between them (e.g. '[0-9]' matches any decimal digit). To include a literal ']' in the sequence, make it the first character (following a possible '^'). To include a literal '-', make it the first or last character.
If a regular expression could match two different parts of the text, it will match the one, which begins earliest. If both begin in the same place but match different lengths, or match the same length in different ways, life gets messier, as follows.
In general, the possibilities in a list of branches are considered in left-to-right order, the possibilities for '*', '+', and '?' are considered longest-first, nested constructs are considered from the outermost in, and concatenated constructs are considered leftmost-first. The match that will be chosen is the one that uses the earliest possibility.
For example, '(ab|a)b*c' could match 'abc' in one of two ways. The first choice is between 'ab' and 'a'; since 'ab' is earlier, and does lead to a successful overall match, it is chosen. Since the 'b' is already spoken for, the 'b*' must match its last possibility-the empty string-since it must respect the earlier choice.
In the particular case where no '|'s are present and there is only one '*', '+', or '?', the net effect is that the longest possible match will be chosen. So 'ab*', presented with 'xabbbby', will match 'abbbb'. Note that if 'ab*' is tried against 'xabyabbbz', it will match 'ab' just after 'x', due to the begins-earliest rule.