regexp_grammar.hpp has almost every grammar element now. The 'mark_tag' ones are missing and the look-ahead assertions need to be fixed.
Modifications on the grammar: qexp recevied the 'no_back_ref' element and bschar the '^', '$' and '.' elements.
Added 26 new test cases to cover the new features.
Enhancing the gen_test.pl script to handle the new test cases.