function checkNotNull(field){
    if (field.value.length > 0 )
      return true;
    else
      return false;
  }
  function checkLength(field, min, max){
    if (field.value.length >= min && field.value.length <= max )
      return true;
    else
      return false;
  }
  function checkNumber(field){
    if (isNaN(field.value))
      return false;
    else
      return true;
  }
  function checkNumberLength(field, min, max){
    return checkLength(field, min, max) && checkNumber(field);
  }
  function isValidEmail(field){
    if (field.value != "") {
      if (    (field.value.indexOf ('@') == -1)
           || (field.value.indexOf ('.') == -1) ){
        return false;
      }
      else
        return true;
    }
    else
      return true;
  }
  function isValidDate(dateString){
    // Tag und Monat koennen einstellig, Jahr muß vierstellig sein: (D[D].M[M].YYYY)
    // Datum darf auch leer sein
    var month_length= new Array(0,31,28,31,30,31,30,31,31,30,31,30,31);
    var point_count=0;
    var points = new Array();
    if(dateString.length==0)
      return true;

    for(i=0;i<dateString.length;i++){
      if(dateString.charAt(i)=="."){
        points[point_count]=i;
        point_count++;
      }
    }
    if(!(point_count!=2)){
      var day=parseInt(dateString.substring(0,points[0]),10);
      var month=parseInt(dateString.substring(points[0]+1,points[1]),10);
      var year=parseInt(dateString.substring(points[1]+1,dateString.length),10);
      var yearString=dateString.substring(points[1]+1,dateString.length);
      if (!(yearString.length!=4||year<0 || year>9999||isNaN(year))){
        if (year%4==0){
          if(!((year%100==0)&&(year%400!=0)))
            month_length[2] = 29;
        }
        if (!(month<1 || month>12 ||isNaN(month))){
          if (!(day<1 || day> month_length[month]||isNaN(day))){
            if (day<10)
              day="0"+ day;
            if (month<10)
              month="0"+ month;
            dateString=day+"."+month+"."+year;
            return true;
          }
        }
      }
    }
    return false;
  }