PHP: Detect invalid characters in a text

Question

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!

Gumbo · Accepted Answer · 2009-06-12 18:12:22Z

3

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.

edited Jun 12, 2009 at 18:12

answered Jun 12, 2009 at 17:47

Gumbo

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12 Comments

caw Over a year ago

Thanks! Unfortunately, it returns an "Unknown modifier" error for lots of characters. At first, the error only appears for "(" but when I strip the "(", then it appears also for other characters. Can I escape them so that it works, though?

Gumbo Over a year ago

The / and ] needed to be escaped.

caw Over a year ago

Thank you! Now I get the message "Compilation failed: invalid UTF-8 string at offset 11". This should be due to "äöüß", shouldn't it? How can I encode these characters?

caw Over a year ago

I use UTF-8. I can't replace the pattern by "Ã¤Ã¶Ã¼ÃŸ", can I?

Gumbo Over a year ago

When you’re using UTF-8 to encode that file, there should be no errors. This error only occurs when your file is not encoded with UTF-8.

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