I've been trying to figure out how to do a recursive regular expression in Perl 6. For a toy example, a balanced parentheses matcher, which would match ((())()) inside (((((())()).
I thought this would do it:
my regex paren {
'(' ~ ')' <paren>*
}
or the simpler
my regex paren {
'(' <paren>* ')'
}
but that fails with
No such method 'paren' for invocant of type 'Match'
in regex paren at ...
You need to make explicit that you're calling a my-scoped regex:
my regex paren {
'(' ~ ')' <&paren>*
}
Notice the & that has been added. With that:
say "(()())" ~~ /^<&paren>$/ # ｢(()())｣
say "(()()" ~~ /^<&paren>$/ # Nil
While it's true that you can sometimes get away without explicitly writing the &, and indeed could when using it:
say "(()())" ~~ /^<paren>$/ # ｢(()())｣
say "(()()" ~~ /^<paren>$/ # Nil
This only works because the compiler spots there is a regex defined in the lexical scope with the name paren so compiles the <paren> syntax into that. With the recursive case, the declaration isn't installed until after the regex is parsed, so one needs to be explicit.
You can use the ~~ in the meta-syntax to make a recursive callback into the current pattern or just a part of it. For example, you can match balanced parenthesis with the simple regex:
say "(()())" ~~ /'(' <~~>* ')'/; # ｢(()())｣
say "(()()" ~~ /'(' <~~>* ')'/; # ｢()｣
Unfortunately, matching via a captured subrule (like ~~0) is not yet implemented.
nonparenas stuff I don't want, and an action class that will collect theparenmatches... but that gets complicated fast... It's just very hard to believe P6 regular expressions dropped support for something Perl basically pioneered.