Check for non-numeric bits in the stem.

lib/spack/spack/url.py

@@ -207,7 +207,7 @@ def parse_version_offset(path):
         (r'-((\d)+-\d)', stem),
 
         # e.g. foobar_1.2-3
-        (r'_((\d+\.)+\d+(-\d+)?)', stem),
+        (r'_((\d+\.)+\d+(-\d+)?[a-z]?)', stem),
 
         # e.g. foobar-4.5.1
         (r'-((\d+\.)*\d+)$', stem),