Codewars, but the only language is regex.
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Said It Twice
Match text where the same word appears twice in a row.
A backreference \1 matches whatever group 1 actually captured — not the pattern again, the text. This is the classic duplicated-word bug your editor highlights.
the the catmust matchhello hellomust matcha amust matchany run of whitespace between them
the catmust not matchthe theatremust not matchtheatre merely starts with the
+ 4 hidden tests, checked when you submit. They are what stops a pattern that only fits the examples above.
Solve it, then shrink it
Passing is the easy half. Your score is the length of your pattern, so the real competition starts once it turns green.
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68 challenges, eight tiers
From matching a literal word to writing the regex that detects prime numbers. Browse them all.