Валентин

preg_replace('/(@[a-zA-Zа-яА-Я0-9]+?)#(\d+)/u', '<a href="/user/$2" class="user-tag">$1</a>', $text);

$url = preg_replace('/page_[0-9]+\//', '', '/catalog/masla-zhidkosti-i-smazki/page_2/');

mysql> SELECT REGEXP_REPLACE('a b c', 'b', 'X');
+-----------------------------------+
| REGEXP_REPLACE('a b c', 'b', 'X') |
+-----------------------------------+
| a X c                             |
+-----------------------------------+

UPDATE `site_content` SET `content` = REGEXP_REPLACE(content, 'title="Иллюстрация [0-9]+"', '')

<meta.*charset=["]?([0-9A-z-]+)["]?[\/]?>

<meta charset="utf-8"> // utf-8
<meta http-equiv="Content-Type" content="text/html; charset=windows-1251"/> // windows-1251

preg_match('~^'.preg_quote($uriPattern, '&').'$~', $this->uri)