ciao grazie per la risposto
ho provato ma non va. non mi dice niente

<html>
<head>
<title>Modulo di registrazione</title>

<script type="text/javascript">


function pass(obj){
var str=obj.value;
if(str.lenght<5){ alert("la password deve essere lunga 5 caratteri");
obj.focus();}
}


</script>


</head>

<body>

<div align="center">
<h1>REGISTRAZIONE UTENTE</h1>
</div>
<FORM NAME="formregistrazione" action="eseguiregistrazione.php" method="post" >
<div align="center">
<table width="540" height="368" border="0" bordercolor="#99CC33">
<tr>
<td width="75">Nome:</td>
<td width="155"><input type="text" name="nome"></td>
<td width="86">Cognome:</td>
<td width="206"><input type="text" name="cognome"></td>
</tr>
<tr>
<td>Indirizzo:</td>
<td colspan="3"><input name="indirizzo" type="text" size="30"></td>
</tr>
<tr>
<td>Citt&agrave;</td>
<td><input name="citt&agrave;" type="text" size="24"></td>
<td> CAP</td>
<td><input name="CAP" type="text" size="6">
</td>
</tr>
<tr>
<td>Paese</td>
<td><select name="paese">
<option>Italia</option>
<option>Francia</option>
<option>Germania</option>
<option>Spagna</option>
<option>Portogallo</option>
</select></td>
<td>Provincia</td>

</select>
Sesso
<select name="sesso">
<option>Maschio</option>
<option>Femmina</option>
</select></td>
</tr>
<tr>
<td>Comune di nascita</td>
<td colspan="3"> C D
<input type="text" name="comunenascita" size="20" value="">
<tr>
<td>Codice fiscale</td>
<td><input name="codicefiscale" type="text" size="24" value=""></td>
<td></td>
<td></td>
</tr>
<tr>
<td>Username:</td>
<td><input type="text" name="username"></td>
<td></td>
<td></td>
</tr>
<tr>
<td>Password:</td>
<td><input type="password" name="password" onBlur="pass(this)"></td>
<td>Conferma Password:</td>
<td><input type="password" name="confermapassword"
onBlur="confronta_campi(password.value, confermapassword.value)"></td>
</tr>
<td colspan="2" align="center"><input type="button" onClick="inviaForm1();" name="action" value="Invia"></td>
</tr>
</table>
</div>
</form>
</body>
</html>