Online PHP editor | output for M8jT2

<?php $str = "příliš žluťoučký img_9 kůn úpěl ďábelské ódy"; var_export(mb_strpos($str, 'img_')); echo "\n"; var_export(preg_match_all('/\G(?!img_\d)\X/u', $str, $m)); echo "\n"; $mbLength = preg_match_all('/\G(?!img_\d)\X/u', $str, $m); var_export(mb_strlen($str) !== $mbLength ? $mbLength : 'not found'); echo "\n"; var_export( preg_match('/\X*?(?=img_\d)/u', $str, $m) ? mb_strlen($m[0]) : 'not found' );