diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -29,7 +29,7 @@ def _findtool(ui, tool):
         k = _toolstr(ui, tool, kn)
         if not k:
             continue
-        p = util.lookup_reg(k, _toolstr(ui, tool, "regname"))
+        p = util.lookupreg(k, _toolstr(ui, tool, "regname"))
         if p:
             p = util.find_exe(p + _toolstr(ui, tool, "regappend"))
             if p:
diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -439,8 +439,8 @@ else:
                     rcpath.append(os.path.join(progrcd, f))
             return rcpath
         # else look for a system rcpath in the registry
-        value = util.lookup_reg('SOFTWARE\\Mercurial', None,
-                                _HKEY_LOCAL_MACHINE)
+        value = util.lookupreg('SOFTWARE\\Mercurial', None,
+                               _HKEY_LOCAL_MACHINE)
         if not isinstance(value, str) or not value:
             return rcpath
         value = value.replace('/', os.sep)
diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -487,7 +487,7 @@ def checkwinfilename(path):
             return _("filename ends with '%s', which is not allowed "
                      "on Windows") % t
 
-def lookup_reg(key, name=None, scope=None):
+def lookupreg(key, name=None, scope=None):
     return None
 
 def hidewindow():
diff --git a/mercurial/win32.py b/mercurial/win32.py
--- a/mercurial/win32.py
+++ b/mercurial/win32.py
@@ -180,7 +180,7 @@ def testpid(pid):
             _kernel32.CloseHandle(h)
     return _kernel32.GetLastError() != _ERROR_INVALID_PARAMETER
 
-def lookup_reg(key, valname=None, scope=None):
+def lookupreg(key, valname=None, scope=None):
     ''' Look up a key/value name in the Windows registry.
 
     valname: value name. If unspecified, the default value for the key