Fix for #2613, 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.
Fix for #2613, 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.
Added extra unit test from the example in the ticket.
Merge pull request #2718 from sndpl/issue/2613

Fix for #2613