// Esto ha salido de http://www.tejedoresdelweb.com/	
// funciones isEmpty, isDigit, isNumber

    // s está vacio
    function isEmpty(s)
    {   return ((s == null) || (s.length == 0))}

    // c es un digito
    function isDigit (c)
    {   return ((c >= "0") && (c <= "9"))}

    var defaultEmptyOK = false;
    // s es un número
    function isNumber(s)
    {   var i;
        var dotAppeared;
        dotAppeared = false;
        if (isEmpty(s)) 
           if (isNumber.arguments.length == 1) return defaultEmptyOK;
           else return (isNumber.arguments[1] == true);
        
        for (i = 0; i < s.length; i++)
        {   
            var c = s.charAt(i);
            if( i != 0 ) {
                if ( c == "." ) {
                    if( !dotAppeared )
                        dotAppeared = true;
                    else
                        return false;
                } else     
                    if (!isDigit(c)) return false;
            } else { 
                if ( c == "." ) {
                    if( !dotAppeared )
                        dotAppeared = true;
                    else
                        return false;
                } else     
                    if (!isDigit(c) && (c != "-") || (c == "+")) return false;
            }
        }
        return true;
}