function text (str) { return /[0-9_;:'!~?=+<|>]/g.test(str); }

   function numeric (str) { return /^[0-9-\+\(\)\s]+z/.test(str + "z"); }

   function mail (str) { return /^[a-z0-9_\.]+@[a-z0-9_\.]+.[a-z]{2,3}$/.test(str); }

   function checkForm () 
      {
      var title;
      var elem;
      var dutyField = "Field is empty ";
      var wrongField = "Field is not correct ";
      var check = true;

      function checkError (field, str) 
         {
         document.getElementById("alert").innerHTML = str;
         document.forms.preview.field.focus();
         check = false;
         }

      document.getElementById("alert").innerHTML = "";


      if (check)
         {
         title = '"Jméno"';
         elem = document.preview.jmeno.value;
         if (elem.length == 0) checkError('jmeno', dutyField + title);
         else if (text(elem)) checkError('jmeno', wrongField + title);
         }
         
      if (check)
         {
         title = '"Pošta"';
         elem = document.preview.posta.value;
         if (elem.length == 0) checkError('posta', dutyField + title);
         else if (!mail(elem)) checkError('posta', wrongField + title);
         }

      if (check)
         {
         title = '"Telefonní číslo"';
         elem = document.preview.tlp.value;
         if (elem.length == 0) checkError('tlp', dutyField + title);
         else if (!numeric(elem)) checkError('tlp', wrongField + title);
         }
             
         
      if (check)  { document.preview.submit(); }

      return check;
      }