re won't raise nor return when working with Runaway Regular Expression.
It will compute "almost" indefinitely. Although I'm pretty sure it *may* complete sometime, it's definetly looks like it's stuck.

```
> python -VVVV
Python 3.6.2 (default, Aug 23 2017, 14:57:08)
[GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.42)]
```

Reproduce with attached file.

Should there be a (configurable?) limit on the number of steps involved in the process.
Or some warnings and/or hard limit that raises exception?

https://pythex.org/ will fail with a HTTP502 BadGateway (server taking too long to respond)
https://regex101.com/ python's tester seems to set a limit for this case. I can't say how they managed this.