Regex to detect invalid UTF-8 string

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In PHP, we can use mb_check_encoding() to determine if a string is valid UTF-8. But that's not a portable solution as it requires the mbstring extension to be compiled in and enabled. Additionally, it won't tell us which character is invalid.

Is there a regular expression (or another other 100% portable method) that can match invalid UTF-8 bytes in a given string?

That way, those bytes can be replaced if needed (keeping the binary information, such as when building a test output XML file that includes binary data). So converting the characters to UTF-8 would lose information. So, we may want to convert:

"foo" . chr(128) . chr(255)

Into

"foo<128><255>"

So just "detecting" that the string is not good enough, we'd need to be able to detect which characters are invalid.

utf-8 validation php regex
4 Answers

This works for me for detecting Unicode characters, linke emoji, Russian or Chinese:

private function has_unicode($string)
{
    $pattern = '/^.*[^\x{00}-\x{00FF}]+.*$/u';
    return preg_match($pattern, $string) ? true : false;
}
user_43853040
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