Fix for #2613, remove unnecessary backslash before each utf-8 char

when using  php < 5.4 there was an unnecessary backslash before each utf-8 char.  The problem was that the regexp matched all utf-8 encoded chars (included the ones that where escaped). The new regexp uses the lookbehind feature to check if the backslash isn't prefixed with an other backslash.

This should fix ticket #2613.