Codewars, but the only language is regex.
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Name The Pair
A hand is five cards as "AS 7H 7D 9C KS" — rank then suit, space separated, and always sorted by rank, ascending. Capture the rank that appears more than once, or match nothing if every rank is different.
Sorted means duplicate ranks are neighbours, so you are comparing one rank with the rank exactly three characters later. The trap is that suits are neighbours too — a lazy pattern will happily report a shared suit as a pair, so make sure your group can only start where a card starts.
AS 7H 7D 9C KS→ ["7"]AS AH 5D 9C KS→ ["A"]2S 2H 2D 9C KS→ ["2"]three of a kind counts
2S 5H 9D TC KSmust not match2S 5S 9S TS KSmust not matchfive spades — a shared suit is not a pair
+ 7 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.