Regexp filter

Allows to do stuff like:

```perl
process $exp, $key => [ 'TEXT', qr/Price: (\d+)/ ];
process $exp, $key => [ 'TEXT', qr/(?<name>\w+): (?<value>\w+)/ ];
```

Named capture groups are wrapped in a *hashref* (they obviously won't work prior to Perl 5.10).