--- a/mercurial/bookmarks.py	Tue Jun 20 14:35:53 2017 -0700
+++ b/mercurial/bookmarks.py	Sat Jun 10 23:32:58 2017 -0700
@@ -19,6 +19,7 @@
     error,
     lock as lockmod,
     obsolete,
+    scmutil,
     txnutil,
     util,
 )
@@ -622,3 +623,15 @@
     else:
         # still an independent clause as it is lazier (and therefore faster)
         return old.descendant(new)
+
+def checkformat(repo, mark):
+    """return a valid version of a potential bookmark name
+
+    Raises an abort error if the bookmark name is not valid.
+    """
+    mark = mark.strip()
+    if not mark:
+        raise error.Abort(_("bookmark names cannot consist entirely of "
+                            "whitespace"))
+    scmutil.checknewlabel(repo, mark, 'bookmark')
+    return mark

--- a/mercurial/commands.py	Tue Jun 20 14:35:53 2017 -0700
+++ b/mercurial/commands.py	Sat Jun 10 23:32:58 2017 -0700
@@ -957,14 +957,6 @@
     rename = opts.get('rename')
     inactive = opts.get('inactive')
 
-    def checkformat(mark):
-        mark = mark.strip()
-        if not mark:
-            raise error.Abort(_("bookmark names cannot consist entirely of "
-                               "whitespace"))
-        scmutil.checknewlabel(repo, mark, 'bookmark')
-        return mark
-
     def checkconflict(repo, mark, cur, force=False, target=None):
         if mark in marks and not force:
             if target:
@@ -1039,7 +1031,7 @@
                     raise error.Abort(_("new bookmark name required"))
                 elif len(names) > 1:
                     raise error.Abort(_("only one new bookmark name allowed"))
-                mark = checkformat(names[0])
+                mark = bookmarks.checkformat(repo, names[0])
                 if rename not in marks:
                     raise error.Abort(_("bookmark '%s' does not exist")
                                       % rename)
@@ -1052,7 +1044,7 @@
                 tr = repo.transaction('bookmark')
                 newact = None
                 for mark in names:
-                    mark = checkformat(mark)
+                    mark = bookmarks.checkformat(repo, mark)
                     if newact is None:
                         newact = mark
                     if inactive and mark == repo._activebookmark: