egrep - LOVE it

'egrep' is an alias for grep invoked with the '-e' switch to enable extended regular expressions.

The reason I especially love it is its ability to use inclusive "or's". In other words, give me all the lines with x, y or z.

So if you want to see a count of all the mails sent to your exim daemon from say, 5 to 8 pm on 9/22, you can simply execute the following:

egrep '(2009-09-22 17|2009-09-22 18|2009-09-22 19|2009-09-22 20)' mainlog.1 | grep dnslookup | wc -l

And voila! You will have your answer.