Panic when stateful lexer's non-Root rule has optional group but captures nothing about participle HOT 3 OPEN

I think this is probably occurring because Participle allows you to use references like \1 in the destination state, to refer to captured groups from the parent state. Because you have a capture group that doesn't capture anything, it's failing. It needs to cater to that case though.

from participle.

Mm actually sorry, I misread. That might not be it.

Try using a non-capturing group and see if that fixes it: (?:[[:alnum:]]*)

from participle.

If we change pattern of ident to [[:alpha:]][[:alnum:]]* (Just remove the inner group, or add ?: make it non-capture), it will works fine.

Yes, change pattern to non-capture will fix this, actually, it is the workaround I'm using in my real project.

from participle.