RegexdrillBackreference: a doubled letter
A regex exercise from Regexdrill. Solve it with a pattern box with live highlighting in your browser, checked against hidden test strings, and it is checked the moment you submit.
The task
A match anywhere counts. Match lines that contain the same lowercase letter twice in a row, like the ll in hello. Only letters count, and matching is case sensitive, so aA and 11 do not count.
What this exercise teaches
([a-z])\1 captures a letter and demands the identical letter next. [a-z]{2} would accept any two letters, and (\w)\1 would also accept doubled digits and underscores.
A hint if you get stuck
Capture one lowercase letter and follow it with \1, the backreference to that capture.
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