Hmm, that's an error importing the regex module. I'm not familiar enough with the code of that module to be able to guess the cause of the error, you'd have to ask here:
https://bitbucket.org/mrabarnett/mrab-regex
However, since the regex module is only used for some small parts of calibre functionality, I will change the next release to only import that module on demand, so most of calibre should still work.