Course of Raku / Regexes and grammars / Grammars / Grammars, classes, and inheritance / Exercises / A number with proto

Solution: A number with proto

Here is a possible solution to the task.

Code

grammar Number {
    token TOP { <number> }

    proto token number {*}
    token number:sym<dec> { \d+ }
    token number:sym<hex> { '0x' <[0..9a..f]>+ }
}

say Number.parse('0xff').defined;

🦋 You can find the source code in the file proto-number.raku.

Output

True

Comments

1. The proto token number has two variants, dec and hex.

2. For 0xff, the dec variant matches only the leading 0, which would leave xff over; the hex variant matches the whole thing. Longest-token matching therefore picks hex, and the string parses. The same grammar still accepts a plain decimal such as 42.

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