Language: PHP
Label: safe
CWE: CWE-79 \u2014 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

Code:
    public function validate($string, $config, $context)
    {
        $string = trim($string);

        // early abort: '' and '0' (strings that convert to false) are invalid
        if (!$string) {
            return false;
        }

        $pattern = '/^(-?[A-Za-z_][A-Za-z_\-0-9]*)$/';
        if (!preg_match($pattern, $string)) {
            return false;
        }
        return $string;
    }