\\ regex

Hi, my regex is just poor, please help me out here

I want to make sure my $string is double slashed as path separator, my $string can be

$string = "C:\temp\folder"
or
$string = "C:\\temp\\folder"

I have this
$string =~ s/\\/\\\\/g;
but it's wrong because the second $string will become
$string = "C:\\\\temp\\\\folder"

How to specify one \ only?

Thanks.