Prova questo....

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function ltrim(item) { 
return item.replace(/^\s*/gi,""); 
} 
function rtrim(item) { 
return item.replace(/\s*$/gi,""); 
} 
function trim(item) {
 return rtrim(ltrim(item));
}

function controlla(aValue) {
 aValue  = trim(aValue);
 pattern = /^mms:[/][/]\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\:\d{4}$/;   
 if (! aValue.match(pattern)) {
  alert("falso");
  return false;
 } // if (! aValue.match(pattern))
 alert("ok"); 
 return true;
} // function controlla(aValue)
//-->
</script>
</head>
<body>
<input type="text" id="testo" size="25">
<input type="button" value="Controlla" onclick="controlla(document.getElementById('testo').value)">

</body>
</html>