So for output you were looking for this?
a=1
b=2\&3
c=4\\\&d=5

[download]
If that is what you are looking for the problem was with your grouping '()' and the \1. The \1 was not matching the (\\) but it was trying to match the *first* grouping which was ((\\)+\1) and I dont think is what you want.
So change the regex to /[^\\]\\(?:(\\)+\1)*$/ But of course you could save a lot of time an just do: my @split = split /(?<!\\)&/, $tosplit; Using the negative lookbehind assertion which is talked about here.

In reply to Re: Backslashes in regular expressions by perlmonkey
in thread Backslashes in regular expressions by nuance

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