JavaScript: Password Validation using regular expressions and HTML5

<script type="text/javascript"> 
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(form.pwd1.value.length < 6) 
{ 
alert("Error: Password must contain at least six characters!"); form.pwd1.focus(); 
return false; 
} 
if(form.pwd1.value == form.username.value) 
{ 
alert("Error: Password must be different from Username!"); 
form.pwd1.focus(); 
return false; 
} 
re = /[0-9]/; if(!re.test(form.pwd1.value)) 
{ 
alert("Error: password must contain at least one number (0-9)!"); form.pwd1.focus(); return false; 
} 
re = /[a-z]/; 
if(!re.test(form.pwd1.value)) 
{ 
alert("Error: password must contain at least one lowercase letter (a-z)!"); form.pwd1.focus(); 
return false; 
} 
re = /[A-Z]/; 
if(!re.test(form.pwd1.value)) 
{ 
alert("Error: password must contain at least one uppercase letter (A-Z)!"); form.pwd1.focus(); 
return false; 
} 
} 
else 
{ 
alert("Error: Please check that you've entered and confirmed your password!"); form.pwd1.focus(); 
return false; 
} 
alert("You entered a valid password: " + form.pwd1.value); 
return true; 
} 
</script> 
<form ... 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>

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>