((\\)+\1)* is not equivalent to (\\\\)* because of the grouping (and the multiplier, but that part is obvious). I think, because you group the entire term ((\\)+\1)) then \1 is not refering to the (\\). I believe \1 would be undefined at that moment because you are actually inside the first pattern which would be ((\\)+\1)).

I am sure you want to use the ?: operator which "is for clustering, not capturing" which is from the perlre perldoc. Using ?:, the outer group will not get reference to \1 or $1, so the (\\) will get referenced to \1. Then (?:(\\)+\1))* should be equivalent to (\\+\\)*

I am sure this is all vague and confusing to most, but I hope it helped a little.