//HTML//
<td><input style="text-align:center" size="1" type="text" name="160[prefix]" onkeydown="UCD(this);" onkeyup="UCF(this);" value="6"></td>

//JAVASCRIPT//

<script language="javascript">
	<!--
	var prefix_array = new Array(1,2,5,6,7,8,9,20,27,30,31,32,33,34,36,39,40,41,43,44,45,46,47,48,49,51,52,53,54,55,56,57,58,61,62,63,64,65,66,81,82,84,86,90,91,92,93,94,95,98,212,213,216,218,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,260,261,262,263,264,265,266,267,268,269,290,291,297,298,299,350,350,351,352,353,354,355,356,357,358,370,371,371,372,373,374,375,376,377,378,379,380,381,382,385,386,387,389,420,421,423,500,501,502,503,504,505,506,507,508,509,590,591,592,593,594,596,597,598,599,850,852,853,855,856,880,886,960,961,962,963,964,965,966,967,968,970,971,972,973,974,975,976,977,992,993,994,995,998,3906);
	var old_val;
	var old_color;

	function UCF(AField)
	{
           var curent_val=trim(AField.value);
           AField.value=curent_val;

	    if (curent_val!=old_val && curent_val!="")
	    {
				  pos = prefix_array.in_array(old_val,true);
				  if (prefix_array.in_array(curent_val,false)) AField.style.color="red";
				  prefix_array.splice(pos,1,curent_val);
			
	    } else AField.style.color=old_color;

	}

        function UCD(AField)
        {
		old_color=AField.style.color;
		AField.style.color="black";

		if (AField.value!="")
		{
			old_val=AField.value;
			return true;
		}	
	}

	function trim(str, chars) {
	    return ltrim(rtrim(str, chars), chars);
	}

	function ltrim(str, chars) {
	    chars = chars || "\\s";
	    return str.replace(new RegExp("^[" + chars + "]+", "g"), "");
	}

	function rtrim(str, chars) {
	    chars = chars || "\\s";
	    return str.replace(new RegExp("[" + chars + "]+$", "g"), "");	
	}

	Array.prototype.in_array = function(p_val,res)
	{
		for(var i = 0, l = this.length; i < l; i++) {
			if(this[i] == p_val) {
			if (res) return i; else return true;
		}	
	}
		return false;
	}

        </script>