What's the meaning of a number after a backslash in a regular expression?

Question

(a|b)\\1

What does \\1 mean in this expression?

Answer 1

\1 - it means the first capturing group in the matched expression. \n would be the nth capturing group. (Note that \0 would 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 match aa or bb.