Regexp::Compare implements a function comparing regular expressions: it returns
true if all strings matched by the first regexp are also matched by the second.
It's meant to be used for optimization of blacklists implemented by regular
expressions (like, for example, http://www.communitywiki.org/cw/BannedContent).

False return value does not imply that there's a string matched by the first
regexp which isn't matched by the second - many regular expressions (i.e. those
containing Perl code) are impossible to compare, and this module doesn't even
implement all possible comparisons.