Validation for username&Password

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index.php:

<form class="form-wrapper" method="post">
<div align="center"><p style="margin-right:186px;">Username<span style="color:red;">*</span></p><input type="text" class="search" name="username" required="required" id="username" autocomplete="off" onblur="checkUserName(this.value)"/>
<p style="margin-right:70px;"><span id="usercheck"></span></p></div>
 <p>&nbsp;</p>
<div align="center"><p style="margin-right:150px;">Create Password<span style="color:red;">*</span></p><input type="password" class="search" autocomplete="off" required pattern="(?=.*\d)(?=.*[a-z])(?=.*[A-Z])\w{6,}" name="pwd1" onchange="this.setCustomValidity(this.validity.patternMismatch ? 'Password must contain at least6 characters, including UPPER/lowercase and numbers' : '');if(this.checkValidity()) form.pwd2.pattern = this.value;
"></p></div>
<p>&nbsp;</p>
<div align="center"><p style="margin-right:150px;">Verifed Password<span style="color:red;">*</span></p> 
<input type="password" class="search" required pattern="(?=.*\d)(?=.*[a-z])(?=.*[A-Z])\w{6,}" name="pwd2" onchange="
  this.setCustomValidity(this.validity.patternMismatch ? 'Please enter the same Password as above' : '');
"></p>  </div><p align="center"> <input type="submit" class="submit" value="Sign up"  name="submit"></p>
 </form>

username.php:

<?php 
$query=mysql_connect("localhost","root","");
mysql_select_db("reg_ajax",$query);
$arr_user=array("itechroom", "trialuser");
$username=$_POST['user_name'];


if(in_array($username,$arr_user)) 
{echo '<span class="error">Username already exists.</span>';exit;}

else if(strlen($username) < 6 || strlen($username) > 15)
{
echo '<span class="error">Username must be 6 to 15 characters</span>';}
else if (preg_match("/^[a-zA-Z1-9]+$/", $username)) 
{
       echo '<span class="success">Username is available.</span>';
} 
else 
{
      echo '<span class="error">Use alphanumeric characters only.</span>';
}

?>

Script:

function checkUserName(usercheck)

{
    $('#usercheck').html('<img src="images.gif" />');
    $.post("username.php", {user_name: usercheck} , function(data)
        {           
               if (data != '' || data != undefined || data != null) 
               {                   
                  $('#usercheck').html(data);   
               }
          });
}