As you noticed, there is currently no effective way to do this with the ASA's regex matching. You would be better off using a more flexible URL blocking/filtering solution, like the CSC-SSM or Websense/Smartfilter.
As a side note, there is an enhancement bug filed (CSCsm89915) to add the "end of string" special character, which would let you accomplish this in the future:
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...