Sample HTML and JavaScript code

<script type="text/javascript"> 
 function checkPassword(str) 
{ 
var re = /^(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{6,}$/; 
return re.test(str); 
} 
function checkForm(form) 
{ 
if(form.username.value == "") 
{ 
alert("Error: Username cannot be blank!"); 
form.username.focus(); 
return false; 
} 
re = /^\w+$/; if(!re.test(form.username.value)) 
{ 
alert("Error: Username must contain only letters, numbers and underscores!"); form.username.focus(); 
return false; 
} 
if(form.pwd1.value != "" && form.pwd1.value == form.pwd2.value) 
{ 
if(!checkPassword(form.pwd1.value)) 
{ 
alert("The password you have entered is not valid!"); 
form.pwd1.focus(); return false; 
} 
} 
else 
{ 
alert("Error: Please check that you've entered and confirmed your password!"); form.pwd1.focus(); 
return false; 
} 
return true; 
} 
</script> 
<form method="POST" action="..." onsubmit="return checkForm(this);"> 
<p>Username: <input type="text" name="username"></p> 
<p>Password: <input type="password" name="pwd1"></p> 
<p>Confirm Password: <input type="password" name="pwd2"></p> 
<p><input type="submit"></p> 
</form>