Impossible parse?

[nitingupta910]

I have a parse rule like:

dotted_alpha = { (ASCII_ALPHA | ".")* ~ ASCII_ALPHA }

to parse strings which can contain any mix of "." and ASCII_ALPHA but the last char must be an ASCII_ALPHA. However, I'm unable to parse this simple rule:

test code

    #[test]
    fn test_dotted_alpha() {
        let s = "a.fa.gs.ab";
        let dotted_alpha = match NTriplesParser::parse(Rule::dotted_alpha, s) {
            Ok(mut da) => da.next().unwrap().as_str(),
            Err(e) => {
                println!("====== Err parsing dotted_alpha: {}", e);
                return;
            }
        };
        println!("got dotted_alpha: {}", dotted_alpha);
        assert_eq!(dotted_alpha, s);
    }

output

====== Err parsing dotted_alpha:  --> 1:1
  |
1 | a.fa.gs.ab
  | ^---
  |
  = expected dotted_alpha

[nitingupta910]

I got it working with:

dotted_alpha = { (ASCII_ALPHA* ~ ".")* ~ ASCII_ALPHA+ }

This seems unintuitive and would be difficult to come up with when trying to translate an EBNF style grammar to PEG.

[dragostis]

PEGs are conceptually different from EBNFs. You can expect PEG to parser anything you could express in a CFG; you just have think in terms of PEG.

PEGs are designed to recognize languages instead of generating them, which is why they are popular in parsers.

[CAD97]

Also, note that your PEG version is different from the BNF one: the BNF has no issue with multiple . in a row, whereas the PEG prohibits it. (edit: I read your grammar incorrectly this is incorrect.)

The literal translation fails because of how PEG works. PEG is greedy: (ASCII_ALPHA|".")* will consume every single (ASCII) alphabetic character and period, and then the trailing ~ ASCII_ALPHA won't have anything left to match.

[dragostis]

Closing this for now. Feel free to reopen it if you have more questions.