The documentation states that Admin -> General -> Regexp's ".. can be used everywhere regular expressions are supported by referring to it's name, prefixed with @, for example, @mycustomregexp." however, it doesn't seem to work with the item web.page.regexp (possibly others, I will test this out too).

Here is an example of my item:
web.page.regexp[server,/path/monitor.display,8080,@SmokeTest]

Forum thread:
http://www.zabbix.com/forum/showthread.php?t=16140