Drush: Watchdog search explained
You could just pass the stdout from Drush as the stdin to grep.

Example:

drush wd-show | grep 'Undefined offset'


As a bonus - If you want the errors in a file, try the following:

drush wd-show | grep 'Undefined offset' &gt; ~/Undefined-errors.txt


This'll put the errors into a text file in your home directory :-)