preg_match and UTF-8 in PHP

Looks like this is a "feature", see http://bugs.php.net/bug.php?id=37391

'u' switch only makes sense for pcre, PHP itself is unaware of it.

From PHP's point of view, strings are byte sequences and returning byte offset seems logical (i don't say "correct").

Try adding this (*UTF8) before the regex:

preg_match('(*UTF8)/H/u', "\xC2\xA1Hola!", $a_matches, PREG_OFFSET_CAPTURE);

Magic, thanks to a comment in https://www.php.net/manual/function.preg-match.php#95828

Although the u modifier makes both the pattern and subject be interpreted as UTF-8, the captured offsets are still counted in bytes.

You can use mb_strlen to get the length in UTF-8 characters rather than bytes:

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