Esempio ...
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"  xmlns:v="urn:schemas-microsoft-com:vml">
  <head>
    <title>test</title>
    <script type="text/javascript"> 
	function TestForBlank(str) {
		var re=/\s./;
		if(re.test(str)) {
			alert("non puoi scegliere una username con spazio");
			return false;
		}
		return true;
	}
    </script>
  </head>
  <body>
	<form onsubmit="return TestForBlank(this.CampoNome.value);" method="GET">
		<input type="text" name="CampoNome" />
		

		<input type="submit" value="Vai" />
	</form>
  </body>
</html>
HTH
Zappa