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

If refers to what was matched in the first set of parentheses, the first group. Subsequent number means subsequent parentheses.

(1|2)(3|4)\1\2

Would match:

1313
1414
2323
2424

Not that if you have nested groups, just count from the opening brace (left brace).

(groupOne(groupTwo)stillOne(groupThree(groupFour)))