diff -r 540b022ae7a9 Lib/test/regrtest.py
--- a/Lib/test/regrtest.py	Thu Nov 06 15:33:30 2014 +0100
+++ b/Lib/test/regrtest.py	Thu Nov 06 15:58:55 2014 +0100
@@ -269,6 +269,8 @@ def _create_parser():
     group.add_argument('-f', '--fromfile', metavar='FILE',
                        help='read names of tests to run from a file.' +
                             more_details)
+    group.add_argument('-c', '--changed', action='store_true',
+                       help='select tests marked as changed by Mercurial')
     group.add_argument('-x', '--exclude', action='store_true',
                        help='arguments are tests to *exclude*')
     group.add_argument('-s', '--single', action='store_true',
@@ -361,7 +363,7 @@ def _parse_args(args, **kwargs):
          findleaks=False, use_resources=None, trace=False, coverdir='coverage',
          runleaks=False, huntrleaks=False, verbose2=False, print_slow=False,
          random_seed=None, use_mp=None, verbose3=False, forever=False,
-         header=False, failfast=False, match_tests=None)
+         header=False, failfast=False, match_tests=None, changed=False)
     for k, v in kwargs.items():
         if not hasattr(ns, k):
             raise TypeError('%r is an invalid keyword argument '
@@ -375,6 +377,8 @@ def _parse_args(args, **kwargs):
 
     if ns.single and ns.fromfile:
         parser.error("-s and -f don't go together!")
+    if ns.single and ns.changed:
+        parser.error("-s and -c don't go together!")
     if ns.use_mp and ns.trace:
         parser.error("-T and -j don't go together!")
     if ns.use_mp and ns.findleaks:
@@ -582,6 +586,19 @@ def main(tests=None, **kwargs):
                 if guts and not guts[0].startswith('#'):
                     tests.extend(guts)
 
+    if ns.changed:
+        from subprocess import Popen, PIPE, DEVNULL
+        env = os.environ.copy()
+        env['HGPLAIN'] = '1'
+        prefix = os.path.join('Lib', 'test') + os.path.sep
+        with Popen(['hg', 'status', '-mn'], stdout=PIPE, stderr=DEVNULL, env=env) as p:
+            tests = p.communicate()[0]
+        tests = [t[len(prefix):-3] for t in tests.decode().splitlines()
+                 if t.startswith(prefix + 'test_')]
+        if not tests:
+            print("No test files changed or not running from checkout, exiting.")
+            sys.exit(0)
+
     # Strip .py extensions.
     removepy(ns.args)
     removepy(tests)