The following is a description of the version of regular expressions used by YA-NewsWatcher's filtering. It is NOT an introduction to regular expressions. If you have some familiarity with regular expressions, and want to know about the syntax recognized by YA-NewsWatcher, read on. If you don't know what regular expressions are, or why or how to use them, you need to look elsewhere for insight.
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REGULAR EXPRESSION SYNTAX
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), a `\' followed by a single character (matching that char- acter), 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 char- acters 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 con- catenated 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--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. (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.)
HISTORY
Both code and manual page were written at U of T. They are intended to be compatible with the Bell V8 regexp(3), but are not derived from Bell code.
BUGS
Empty branches and empty regular expressions are not portable to V8.
The restriction against applying `*' or `+' to a possibly null operand is an artifact of the simplistic implementation.
Does not support egrep's newline-separated branches; neither does the V8 regexp(3), though.
Due to emphasis on compactness and simplicity, it's not strikingly fast. It does give special attention to handling simple cases quickly.
