Function to validate letters (incl. special chars) and numbers and to check the min/max size

validate.php

<?php
    /*
        Allowed characters:
        
        ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿ
        \x{00C0}-\x{00CF}\x{00D0}-\x{00D6}\x{00D8}-\x{00DF}\x{00E0}-\x{00EF}\x{00F0}-\x{00F6}\x{00F8}-\x{00FF}

        [space] \n - , . / ! % & ' ( ) : ; ? @ _ £
        \ \n\-\x{002C}\x{002E}\x{002F}\x{0021}\x{0025}-\x{0029}\x{003A}\x{003B}\x{003F}\x{0040}\x{005F}\x{00A3}
    */
    function validate_letters_and_numbers_only($input, $max_size = '', $min_size = ''){
        if(preg_match('/^[0-9a-zA-Z\x{00C0}-\x{00CF}\x{00D0}-\x{00D6}\x{00D8}-\x{00DF}\x{00E0}-\x{00EF}\x{00F0}-\x{00F6}\x{00F8}-\x{00FF}\ \n\-\x{002C}\x{002E}\x{002F}\x{0021}\x{0025}-\x{0029}\x{003A}\x{003B}\x{003F}\x{0040}\x{005F}\x{00A3}]+$/u', $input)){
            //Some special chars take up more space in a database than a single normal char such as 'x'.
            //ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿ£
            $input = preg_replace('/[\x{00C0}-\x{00CF}\x{00D0}-\x{00D6}\x{00D8}-\x{00DF}\x{00E0}-\x{00EF}\x{00F0}-\x{00F6}\x{00F8}-\x{00FF}\x{00A3}]/u', 'xx', $input);

            if(is_numeric($min_size)){
                if(strlen($input) < $min_size)
                    return false;
            }

            if(is_numeric($max_size)){
                if(strlen($input) > $max_size)
                    return false;
            }

            return true;
        }

        return false;
    }
    
    //echo (validate_letters_and_numbers_only('Hello World!', 12)) ? 'Valid' : 'Invalid';