You can attempt to guess the missing characters. There are only 1296 possible combinations...
From iTunes Store: Invalid, inactive, or illegible codes: http://support.apple.com/kb/TS1292:
If you're still unable to redeem your code after verifying the characters in the code, you'll need to contact iTunes Store Support for help. You'll need to provide the activation code and serial number from the back of your iTunes Gift Card.
Serial Number Formats Starts with: Length GCA 16 digits 60 16 or 19 digits EPY 14 digits PBH 22 digits