Why Does PREG_OFFSET_CAPTURE Return Byte Counts Instead of Character Counts with UTF8 and the 'u' Modifier?

PREG_OFFSET_CAPTURE Counting Bytes Despite UTF8 Expression

When utilizing preg_match with the u modifier for UTF8 processing, one might encounter an unexpected behavior where PREG_OFFSET_CAPTURE returns byte counts instead of character counts.

preg_match('/H/u', "\xC2\xA1Hola!", $a_matches, PREG_OFFSET_CAPTURE);
echo $a_matches[0][1]; // Prints 2, but should be 1 for "H" in "¡Hola!"

Despite the u modifier flagging the pattern and subject as UTF8-encoded, the offsets remain in bytes. To obtain character-based offsets, you can employ mb_strlen:

$str = "\xC2\xA1Hola!";
preg_match('/H/u', $str, $a_matches, PREG_OFFSET_CAPTURE);
echo mb_strlen(substr($str, 0, $a_matches[0][1])); // Prints 1

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