19Apr/110
The Alteration Operator
Using egrep, the following will match all lines from the 9th, 10th, 11th, and 12th hours of Sunday April 10th in the log file eventdump.txt:
$ egrep Sun\ Apr\ 10\ \('09|10|11|12'\)\: eventdump.txt
(I always forget to escape the parenthesis.)