function checkField(field,pattern){
		
	var value	= field.value;
	var letter	= "";
	var text	= "";

	for(var i = 0; i < value.length; i++){
		letter	= value.charAt(i);

		if(letter.search(pattern) == 0){
			text += letter;
		}
	}
	
	field.value	= text;
}