Refactor regular expression to avoid expensive backtracking on contrived entry points.

Fixes #361.

Credit to Tim Peters.

Reduces cost from almost 2 seconds to ~100 µs on my workstation as reported by entrypoint_regexp_perf.

Author: jaraco

importlib_metadata/__init__.py

@@ -77,8 +77,8 @@ class EntryPoint(
 
     pattern = re.compile(
         r'(?P<module>[\w.]+)\s*'
-        r'(:\s*(?P<attr>[\w.]+))?\s*'
-        r'(?P<extras>\[.*\])?\s*$'
+        r'(:\s*(?P<attr>[\w.]+)\s*)?'
+        r'((?P<extras>\[.*\])\s*)?$'
         )
     """
     A regular expression describing the syntax for an entry point,