Course of Raku / Regexes and grammars / Grammars / Creating grammars / Exercises / Parse a hashtag

Solution: Parse a hashtag

Here is a possible solution to the task.

Code

grammar Hashtag {
    token TOP { '#' <tag> }
    token tag { \w+ }
}

say Hashtag.parse('#raku')<tag>;

🦋 You can find the source code in the file parse-hashtag.raku.

Output

｢raku｣

Comments

1. TOP spells out the fixed # followed by the <tag> token.

2. .parse requires the whole string to match, and the tag is then available as the <tag> capture.

An alternative

You can instead let tag match the whole hashtag — the # together with the word — and reach the word through a nested word rule:

grammar Hashtag {
    token TOP  { <tag> }
    token tag  { '#' <word> }
    token word { \w+ }
}

say Hashtag.parse('#raku')<tag><word>;

This prints the same ｢raku｣. Now <tag> captures the entire #raku, while the word inside it is reached as <tag><word>. The grammar reads a little more like the thing it describes — a hashtag is a # followed by a word, and the word is a named piece in its own right.

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