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(a|b)\1 What does \1 mean in this expression?
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The backreference \1 (backslash one) references the first capturing group. \1 matches the exact same text that was matched by the first capturing group. The / before it is a literal character.
$ means "Match the end of the string" (the position after the last character in the string).
2. A single backslash means escape, so a second one after it gets escaped, meaning the double backslash matches a single backslash.
\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.
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