PHP: Detect invalid characters in a text

Question

Hello!

I would like to parse user inputs with PHP. I need a function which tells me if there are invalid characters in the text or not. My draft looks as follows:

<?php
function contains_invalid_characters($text) {
    for ($i = 0; $i < 3; $i++) {
        $text = html_entity_decode($text); // decode html entities
    } // loop is used for repeatedly html encoded entities
    $found = preg_match(...);
    return $found;
}
?>

The function should return TRUE if the input text contains invalid characters and FALSE if not. Valid characters should be:

a-z, A-Z, 0-9, äöüß, blank space, "!§$%&/()=[]\?.:,;-_

Can you tell me how to code this? Is preg_match() suitable for this purpose? It's also important that I can easily expand the function later so that it includes other characters.

I hope you can help me. Thanks in advance!

Answer 1

You could use a regular expression to do that:

function contains_invalid_characters($text) {
    return (bool) preg_match('/[a-zA-Z0-9äöüß "!§$%&\/()=[\]\?.:,;\-_]/u', $text);
}

But note that you need to encode that code with the same encoding as the text you want to test. I recommend you to use UTF-8 for that.