Online PHP editor | refs for tZKOF

<?php $string='bla bla xxx bla *xxx bla bla **xxx bla bla ***xxx bla bla ****xxx'; $search='xxx'; $char='*'; $string = preg_replace_callback( '/(' . preg_quote($char) . '*)(' . $search . ')/', function($matches) { global $char; if(strlen($matches[1]) % 2 === 0) { return $matches[1] . $char . $matches[2]; } else { return $matches[0]; } }, $string ); print $string;