Ah, then you need what's called a zero-width negative look-behind. 

$str =~ s/(?<!\\)\\$/\\\\/;  # match a backslash at the EOL
                             # that isn't preceded by a backslash

[download]

See perlre for more stuff like this.

In reply to Re^3: Regex to dereference by gaal
in thread Regex to dereference by Hena

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