This is a function to convert byte offsets into (UTF-8) character offsets (this is reagardless of whether you use /u modifier:

<?php

function mb_preg_match_all($ps_pattern, $ps_subject, &$pa_matches, $pn_flags = PREG_PATTERN_ORDER, $pn_offset = 0, $ps_encoding = NULL) {
  // WARNING! - All this function does is to correct offsets, nothing else:
  //
  if (is_null($ps_encoding))
    $ps_encoding = mb_internal_encoding();

  $pn_offset = strlen(mb_substr($ps_subject, 0, $pn_offset, $ps_encoding));
  $ret = preg_match_all($ps_pattern, $ps_subject, $pa_matches, $pn_flags, $pn_offset);

  if ($ret && ($pn_flags & PREG_OFFSET_CAPTURE))
    foreach($pa_matches as &$ha_match)
      foreach($ha_match as &$ha_match)
        $ha_match[1] = mb_strlen(substr($ps_subject, 0, $ha_match[1]), $ps_encoding);
    //
    // (code is independent of PREG_PATTER_ORDER / PREG_SET_ORDER)

  return $ret;
  }

?>