function checkEmail(adr) {
  var rx = /^([^\s@,:"<>]+)@([^\s@,:"<>]+\.[^\s@,:"<>.\d]{2,}|(\d{1,3}\.){3}\d{1,3})$/;
  var part = adr.value.match(rx);
  var info = 'E-Mail-Adresse ';
  if(part) info += '&lt;'+part[1]+'@'+part[2]+'&gt; ist OK.';
  else { info += 'ist fehlerhaft!'; adr.focus(); }
  document.getElementById('checkEmail').innerHTML = info;
}

document.writeln('<p id="checkEmail" style="margin-bottom:0">Testen Sie das Suchmuster mittels Javascript:</p>');
document.writeln('<form action="" style="margin-top:0"><p style="margin-top:0">');
document.writeln('<input type="text" name="adr" size="35" value="" />');
document.writeln('<input type="button" value="checkEmail" onclick="checkEmail(this.form.adr)" />');
document.writeln('</p></form>');