how can i add alert message before submitting the page

Before submitting the page i need some alert at the password atleast one special cheracter message . How can i write the code.

<?php 

	include 'connect.php';
	include 'fun.php';
	
	$error = "";
	
	if(isset($_POST['savepass']))
	{
		$password = $_POST['password'];
		$confirmPassword = $_POST['passwordConfirm'];
		
		if(strlen($password) < 5)
		{
			$error = "Password must be greater than 5 characters";
		}
		else if($password !== $confirmPassword)
		{
			$error = "Password does not match";
		}
		else if (!preg_match('/[\'^£$%&*()}{@#~?><>,|=_+¬-]/', $password))
		{
			$error = "Atleast one special character in the Password";
		}

		else
		{
			/*
			$password = password_hash($password,PASSWORD_DEFAULT);
			*/
			
			$emp_id = $_SESSION['emp_id'];
			if(mysqli_query($con, "UPDATE users SET password='$password' WHERE EmployeeID='$emp_id'"))
			{
				$error = "Password Changed Successfully, <a href='login2.php'>click here</a> to Log In";
			}
			
		}
	}
	
	
	if(logged_in())
	{
		
		
	?>
		
		<?php echo $error; ?>

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