Course of Raku / Regexes and grammars / Regexes / Alternations

Longest token and first match

When two alternatives could both match at the same spot, the | operator chooses the one that matches the longest piece of text. This is called longest-token matching:

say 'catdog' ~~ / cat | catdog /; # ｢catdog｣

Even though cat is written first and would match, | prefers the longer catdog.

Sometimes you want the opposite — try the alternatives in the order you wrote them and take the first one that matches. That is what the double bar || does:

say 'catdog' ~~ / cat || catdog /; # ｢cat｣

Now cat is tried first and matches, so the engine stops there and never considers catdog.

Use | (longest token) when you want the “best” match regardless of order — this is the usual choice, and the one grammars rely on. Use || (first match) when the order of the alternatives is meaningful and you want earlier ones to win.

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