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 the input string),
- `$' (matching the null string at the end of the input string),
- an `\' (escape char) 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.
AMBIGUITY
If a regular expression could match two different parts of the input string,
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 in the first choice that has to be made.
If there is more than one choice, the next will be made in the same manner
(earliest possibility) subject to the decision on the first choice.
And so forth.
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\(emthe empty string\(emsince
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.
(In effect, the decision on where to start the match is the first choice
to be made, hence subsequent choices must respect it even if this leads them
to less-preferred alternatives.)
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-- Jacques JAMAIN - jamain@ibm.net ... on the french riviera ...