As a hint - your professor is testing your grasp of patterns... this is a really important part of Perl that you'll eventually get to know much better as you get a chance to play around with it. You should read and go through perlre - it will tell you exactly how to approach this problem. As GrandFather said, once you take a stab at it, everyone here will be more than ready to help you with tweeks, confusions, and modifications.

Good luck!
  -Adam

P.S. As another hint: look in the man page for what represents a word boundry, and what it means for characters to be placed within square brackets.