Error in MaskInput Javascript

I'm having problems in a field of a form that is masked through the script below.

MaskInput(document.getElementById('xxx'),'9999');

According to the function it should format the 4 characters of the field in numeric format and accepting any number from 0-9.

The problem is that when using IE 9+ is accepting only the character 8, that is, does not accept any other number.

Any idea what might be happening?

        MaskInput = function(f, m){
       function mask(e){
           var patterns = {"1": /[A-Z]/i, "2": /[0-9]/, "4": /[\xC0-\xFF]/i, "8": /./ },
               rules = { "a": 3, "A": 7, "9": 2, "C":5, "c": 1, "*": 8};
           function accept(c, rule){
               for(var i = 1, r = rules[rule] || 0; i <= r; i<<=1)
                   if(r & i && patterns[i].test(c))
                       break;
                   return i <= r || c == rule;
           }
           var k, mC, r, c = String.fromCharCode(k = e.key), l = f.value.length;
           (!k || k == 8 ? 1 : (r = /^(.)\^(.*)$/.exec(m)) && (r[0] = r[2].indexOf(c) + 1) + 1 ?
               r[1] == "O" ? r[0] : r[1] == "E" ? !r[0] : accept(c, r[1]) || r[0]
               : (l = (f.value += m.substr(l, (r = /[A|9|C|\*]/i.exec(m.substr(l))) ?
               r.index : l)).length) < m.length && accept(c, m.charAt(l))) || e.preventDefault();
       }
       for(var i in !/^(.)\^(.*)$/.test(m) && (f.maxLength = m.length), {keypress: 0, keyup: 1})
           addEvent(f, i, mask);
    };

    addEvent = function(o, e, f, s){
        var r = o[r = "_" + (e = "on" + e)] = o[r] || (o[e] ? [[o[e], o]] : []), a, c, d;
        r[r.length] = [f, s || o], o[e] = function(e){
            try{
                (e = e || event).preventDefault || (e.preventDefault = function(){e.returnValue = false;});
                e.stopPropagation || (e.stopPropagation = function(){e.cancelBubble = true;});
                e.target || (e.target = e.srcElement || null);
                e.key = (e.which + 1 || e.keyCode + 1) - 1 || 0;
            }catch(f){}
            for(d = 1, f = r.length; f; r[--f] && (a = r[f][0], o = r[f][1], a.call ? c = a.call(o, e) : (o._ = a, c = o._(e), o._ = null), d &= c !== false));
            return e = null, !!d;
        }
    };

    removeEvent = function(o, e, f, s){
        for(var i = (e = o["_on" + e] || []).length; i;)
            if(e[--i] && e[i][0] == f && (s || o) == e[i][1])
                return delete e[i];
        return false;
    };