The changes in 84d67af made the regexes accept more inputs as valid than
originally intended due to the use of PCRE shorthands (\w, \d, etc.).
This commit partially reverts to the original and adds the ASCII flag to
`re.compile()` to ensure only characters in the ASCII range are
accepted for the shorthands.

The epoch number in the NEVR form appears just after the package name
so stripping any text before it completely breaks the system.

After this change, the first number followed by a colon (if any such is
present) is removed, since this must be the epoch.

* Refactor control flow in a Pythonic style.
* Replace a needless list comprehension with a generator.
* Use format string literals.