http://pastebin.com/Q8W2LWPt

form-validation.php

<?php

// <form name="register" ...>
if($_POST['register'])
{
  $email_regex = '/^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$/';

	$validity = array(
		'password_ok' => preg_match("/^[a-z0-9_-]*$/i", $pass) 
					 && (strlen($pass < 6) || strlen($pass > 30)),
		'password_match' => ($_POST['pass'] === $_POST['rpass']),
		'username_valid' => preg_match("/^[a-z0-9._-]*$/i", ($_POST['user'])),
		'email_valid' => preg_match($email_regex,  $_POST['email'])
	);

	$errors = array();

	foreach ($validity as $key => $is_valid) 
	{
		if(!$is_valid)
		switch ($key) {
			case 'password_ok':
					$errors[] = 'Invalid Password';
				break;
			case 'password_match':
					$errors[] = 'Password do not match';
				break;
			case 'username_valid':
					$errors[] = 'Invalid Username';
				break;
			case 'email_valid':
					$errors[] = 'Invalid Email';
				break;
		}
	}
	if( count($errors)==0 )
		//insert to database
}

// HTML
<?
if( count($errors)>1 )
	foreach ($errors => $error) 
		echo $error.'<br/>';
?>