1) $preg_match => preg_match
2) Подправил немного твой вариант

$content = '<h1 id="title"><b id="profile_online_lv" class="fl_r">Online<b class="mob_onl profile_mob_onl unshown" id="profile_mobile_online" onmouseover="mobileOnlineTip(this, {mid: cur.oid, right: 1})" onclick="mobilePromo(); ">';

preg_match('/<h1 id="title"><b id="profile_online_lv" class="fl_r">([^<]*)<.*>/', $content, $match);
print_r($match);

Выдает:
Array
(
[0] => <h1 id="title"><b id="profile_online_lv" class="fl_r">Online<b class="mob_onl profile_mob_onl unshown" id="profile_mobile_online" onmouseover="mobileOnlineTip(this, {mid: cur.oid, right: 1})" onclick="mobilePromo(); ">
[1] => Online
)

3) Строго не судить. Сам новичок.