PHPhelp

Im having problems with a signup form I'm trying to create. It's for a class assignment - so don't give me code, but help me understand where I'm going wrong! I'm trying to make a signup form, that checks to see if the username exists, if it does prints a message. It's also meant to check it the password matches certain requirements ( Lowercase, Capital,Number and 6 Characters) I use preg_match but it always says the password doesn't meet the requirements. If it meets the requirements of both the username not existing and the password meeting the length it gets put into the database.

My problem right now is that no matter what I do, the matching says it doesn't meet the requirements, or it won't let me fix it without reloading the the signup form page.

Is there a different way I should go about this? I'm going to update everything to prepared statements after I figure out the logic for the signup.

<form method="POST"> <div class="form-account"> <label for="username">Username</label> <input type="username" class="form-control" id="username" name="username"> </div> <div class="form-account"> <label for="password">Password</label> <input type="password" class="form-control" id="password" name="password"> </div> <input type="submit" name="signsubmit" value="Sign Up" class="btn btn-primary"> <?php $username=""; $password="";

        if(isset($_POST['signsubmit'])){
            $username = $_POST['username'];
            $password = $_POST['password'];

            if(empty($username)){
                echo "Enter a username";
            }
            if(empty($password)){
                echo"Enter a password";
            }

            //See if username exists 
            else{
                $sql="SELECT Login.username FROM Login WHERE Login.username='$username'";
                $result = $conn->query($sql);
                if($result->num_rows > 0){
                    echo "Username exists";
                    }

                else if(!preg_match("(?=[^A-Z]*[A-Z])(?=[^a-z]*[a-z])(?=\D*\d).{6,}", $password)){ ?>
                    <div id="message">
                      <h3>Password must contain the following:</h3>
                          <p>A <b>lowercase</b> letter</p>
                          <p>A <b>capital (uppercase)</b> letter</p>
                          <p>A <b>number</b></p>
                          <p>Minimum <b>6 characters</b></p>  
                    </div>
                 <?php }

                else{
                    session_start();

//input information into DB have other code that I have removed for this.

        }
        }