It's pretty easy (to the extent that implementing custom parsing logic is ever easy) to add custom parsing bits to a Perl 6 grammar. The grammar is just a spec

It's pretty easy (to the extent that implementing custom parsing logic is ever easy) to add custom parsing bits to a Perl 6 grammar. The grammar is just a special class with the rules as methods, so rules with non-standard parsing logic can be implemented using Perl 6 code. Reusing standard rules from custom rules is simply a matter of calling the corresponding method. The best example of this is probably the EXPR rule in the NQP compiler (which is in turn used by Rakudo's parser), which inserts an operator precedence parser into the recursive descent of the rest of the grammar.