Regex: Do not match if pattern at the end of the string

<b> <i> <u> </b> </i> </u>

string = Regex.Replace(string, ".((<[/biu]+>)+)", ".$1||")

This <b>should match.</b> allright.

This <i><b>shouldn't match.</b></i>

".((<[/biu]+>)+)(?!$)"

.(?>(?:</?[biu]>)+)(?!$)

".((<[/biu]+>)+)[^<>]+"