/**
 * Checks the validity of the contact form.
 */
function checkContactForm() {
  if(document.getElementById("firstname").value.length == 0) {
    alert("Please enter your first name.");
    document.getElementById("firstname").focus();
    return false;
  }

  var email = document.getElementById("email").value;
  if(email.length == 0) {
    alert("Please enter your email address.");
    document.getElementById("email").focus();
    return false;
  }
  if(email.match(/^[a-zA-Z0-9\.\-+_]+@[äöüÄÖÜßa-zA-Z0-9\.\-_]+$/) == null) {
    alert("This email address is not valid.");
    document.getElementById("email").focus();
    return false;
  }

  if(document.getElementById("msgtext").value.length == 0) {
    alert("Please enter a message text.");
    document.getElementById("msgtext").focus();
    return false;
  }

  return true;
}