See the CHANGELOG entry here: https://github.com/rust-lang/regex/releases/tag/0.2.7
The most obvious user facing changes are:
- Better error messages.
- Better support for Unicode properties.
- Set difference and symmetric difference operations are now available in character classes.
- Support for Unicode is now exhaustively documented.
There are more details on the specifics of the regex-syntax rewrite here: https://github.com/rust-lang/regex/commit/715a8072890af65d2095d39f534b4b3dc4caeae2
Given the size of the change, it is not impossible that new bugs have been introduced. But the size of the test suite is just as big. Nevertheless, be on the lookout for bugs and please report them!