http://www.perlmonks.org?node_id=639579


in reply to perl regex referencing

It's not a problem to use \1 and friends in the LHS of the substitution because it doesn't behave like a quoted string : the <backslash-digit> token has a well defined meaning there. Moreover, $1 can already be a capture in a previous successful regexp, so it shouldn't be reset before the end of the matching part of the substitution.