"Zero or One" rule

[tomdavsmi]

Hi!

I've been trying to get to grips with lexy by writing a chemical formula parser, and I can't quite
figure out how to emulate the ? construct from regular expressions (i.e. zero or one). I would need this to make "H" or "Fe" to parse as correct, but "Hee" not.

I'm quite new to this kind of thing so there is absolutely a chance that I've just glossed over/not understood the right bit of the documentation.

Thank you!

[foonathan]

You can use either dsl::if_() or dsl::opt() for that. The latter gives you a value when the rule didn't match, the former doesn't.

Although, if you're having a fixed list of symbols, it might make sense to use dsl::symbol: you specify a mapping of e.g. element names to enum values in a table, then read one of those.

[tomdavsmi]

Ahh okay, I'll go and have a play around to try and get more familiar with how branching works. And you're absolutely right, a fixed list of symbols is a much better idea!

Thanks again.