Can I use run Bitcoin Core in regtest mode in v0.8.1?
Regtest mode was introduced in version 0.9.0, via pull request #2632. That is why it doesn't work in 0.8.1, as you noticed. There is no other option to run regtest in that version, although testnet3 is supported by that version so you should still be able to use that if it suits your needs. Otherwise just upgrade to a newer version which will contain performance improvements, bugfixes and newer feature support :)