Logfile monitoring with regular expression is ok. But sometimes you need to search for log entries in more than one line.
More information in this thread: http://www.zabbix.com/forum/showthread.php?t=19641&highlight=Application

Example:

foo
foofoofoo
foofoofoofoo
error and much more
here is the description
foofoofoo
foofoofoofoo

regular expression:

/^error.*\n.*$/m

With this regular expression you will get both lines. But today it is not possible in zabbix