Documentation:
- mention all the rules which are overridden, since it may 
cause incompatibilities with other extensions which do the same

Compatibility:
- (maybe) add an option for using another keyword than "match", and 
avoid to have to delete existing rules

Features:
- port to Str the macros which already exist for PCRE (if possible)

Possible optimizations:
- computation of only the substrings that are effectively used (or at 
  least which name appears in some expression)
