Question

I’ve found this regex in a script I’m customizing. Can someone tell me what its doing?

function test( $text) {
    $regex = '/( [\x00-\x7F] | [\xC0-\xDF][\x80-\xBF] | [\xE0-\xEF][\x80-\xBF]{2} | [\xF0-\xF7][\x80-\xBF]{3} ) | ./x';
    return preg_replace($regex, '$1', $text);
}

Answer 1

Inside of the capturing group there are four options:

1. [\x00-\x7F]
2. [\xC0-\xDF][\x80-\xBF]
3. [\xE0-\xEF][\x80-\xBF]{2}
4. [\xF0-\xF7][\x80-\xBF]{3}

If none of these patterns are matched at a given location, then any character will be matched by the . that is outside of the capturing group.

The preg_replace call will iterate over $text finding all non-overlapping matches, replacing each match with whatever was captured.

There are two possibilities here, either the entire match was inside the capturing group so the replacement doesn’t change $text, or the . at the end matched a single character and that character is removed from $text.

Here are some basic examples:

• If a character in the range \xF8-\xFF appears in the text, it will always be removed
• A character in \xC0-\xDF will be removed unless followed by a character in \x80-\xBF
• A character in \xE0-\xEF will be removed unless followed by two characters in \x80-\xBF
• A character in \xF0-\xF7 will be removed unless followed by three characters in \x80-\xBF
• A character in \x80-\xBF will be removed unless it was matched as a part of one of the above cases