\Q \E is a sequence which allow you to escape whatever is inside this sequence, in this example it is used to escape the dot '.', the forward slash '/' and the '?' question mark, a backward slash could have been used to escape these: /youtube\.com\/watch\?v=(\w+)$/
the /g modifier allows you to multiple match at once, that's why you get 2 in your cat|dog example, if you don't use /g, once one match has been found (cat), nothing else is processed and the function would return 1.
