--- a/python/mozboot/mozboot/base.py
+++ b/python/mozboot/mozboot/base.py
@@ -316,31 +316,36 @@ class BaseBootstrapper(object):
         self.package_manager_updated = True
 
     def _update_package_manager(self):
         """Updates the package manager's manifests/package list.
 
         This should be defined in child classes.
         """
 
-    def _parse_version(self, path, name=None):
+    def _parse_version(self, path, name=None, env=None):
         '''Execute the given path, returning the version.
 
         Invokes the path argument with the --version switch
         and returns a LooseVersion representing the output
         if successful. If not, returns None.
 
         An optional name argument gives the expected program
         name returned as part of the version string, if it's
         different from the basename of the executable.
+
+        An optional env argument allows modifying environment
+        variable during the invocation to set options, PATH,
+        etc.
         '''
         if not name:
             name = os.path.basename(path)
 
         info = self.check_output([path, '--version'],
+                                 env=env,
                                  stderr=subprocess.STDOUT)
         match = re.search(name + ' ([a-z0-9\.]+)', info)
         if not match:
             print('ERROR! Unable to identify %s version.' % name)
             return None
 
         return LooseVersion(match.group(1))
 
@@ -357,25 +362,20 @@ class BaseBootstrapper(object):
         return env
 
     def is_mercurial_modern(self):
         hg = self.which('hg')
         if not hg:
             print(NO_MERCURIAL)
             return False, False, None
 
-        info = self.check_output([hg, '--version'], env=self._hgplain_env()).splitlines()[0]
-
-        match = re.search('version ([^\+\)]+)', info)
-        if not match:
-            print('ERROR: Unable to identify Mercurial version.')
+        our = self._parse_version(hg, 'version', self._hgplain_env())
+        if not our:
             return True, False, None
 
-        our = LooseVersion(match.group(1))
-
         return True, our >= MODERN_MERCURIAL_VERSION, our
 
     def ensure_mercurial_modern(self):
         installed, modern, version = self.is_mercurial_modern()
 
         if modern:
             print('Your version of Mercurial (%s) is sufficiently modern.' %
                   version)