(a|b)\1
What does \1 mean in this expression?
(a|b)\1 What does \1 mean in this expression?
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\1– it means the first capturing group in the matched expression.\nwould be the nth capturing group. (Note that\0would be whole match). In many engines, the upperlimit for n is 9, but some support up to 99 as well.When used in regex like
(a|b)\1, it means that after a or b, the next character should be the first captured group, which is a or b so the regex here would matchaaorbb.