<?php
$str = 'some_string\\\\';
var_dump(preg_replace("/(\/\/)/", "", $str));

$str = '//some_string\\\\';
var_dump(preg_replace("/(\/\/)/", "", $str));

$str = 'some_string\\\\';
var_dump(preg_replace("//", addslashes($str), "hi"));

$str = 'some_string\\\\';
var_dump(addslashes(preg_replace("//", $str, "hi")));