Course of Raku / Regexes and grammars / Regexes / Lookaround assertions / Exercises / Not followed by

Solution: Not followed by

Here is a possible solution to the task.

Code

say so '50 dollars' ~~ / \d+ >> <!before '%'> /;

🦋 You can find the source code in the file not-followed.raku.

Output

True

Comments

1. <!before '%'> is a negative lookahead: it succeeds only when the text right after the number is not a %.

2. The >> is a word boundary marking the end of the number, and it matters here. Without it, the greedy \d+ would backtrack: on 50% off it would give up the 0 and match just 5 — which is not followed by % — so the pattern would wrongly succeed. >> forces \d+ to take the whole number, so the lookahead is tested at the real end. As a result 50 dollars gives True, while 50% off correctly gives False.

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