diff --git a/lib/spack/spack/version.py b/lib/spack/spack/version.py
index d4deee78a9..321a03efc7 100644
--- a/lib/spack/spack/version.py
+++ b/lib/spack/spack/version.py
@@ -37,7 +37,10 @@
 __all__ = ['Version', 'VersionRange', 'VersionList', 'ver']
 
 # Valid version characters
-VALID_VERSION = r'[A-Za-z0-9_.-]'
+VALID_VERSION = re.compile(r'[A-Za-z0-9_.-]')
+
+# regex for version segments
+SEGMENT_REGEX = re.compile(r'[a-zA-Z]+|[0-9]+')
 
 # Infinity-like versions. The order in the list implies the comparison rules
 infinity_versions = ['develop', 'main', 'master', 'head', 'trunk']
@@ -97,9 +100,10 @@ class Version(object):
     """Class to represent versions"""
 
     def __init__(self, string):
-        string = str(string)
+        if not isinstance(string, str):
+            string = str(string)
 
-        if not re.match(VALID_VERSION, string):
+        if not VALID_VERSION.match(string):
             raise ValueError("Bad characters in version string: %s" % string)
 
         # preserve the original string, but trimmed.
@@ -107,12 +111,11 @@ def __init__(self, string):
         self.string = string
 
         # Split version into alphabetical and numeric segments
-        segment_regex = r'[a-zA-Z]+|[0-9]+'
-        segments = re.findall(segment_regex, string)
+        segments = SEGMENT_REGEX.findall(string)
         self.version = tuple(int_if_int(seg) for seg in segments)
 
         # Store the separators from the original version string as well.
-        self.separators = tuple(re.split(segment_regex, string)[1:])
+        self.separators = tuple(SEGMENT_REGEX.split(string)[1:])
 
     @property
     def dotted(self):