diff --git a/test/test_util.py b/test/test_util.py
index 93c4b7ee6..2cfe2f5cd 100644
--- a/test/test_util.py
+++ b/test/test_util.py
@@ -83,15 +83,24 @@ class TestUsernameGenerator(unittest.TestCase):
   def test_basic_ascii_names(self):
     self.assert_generated_output('jake', 'jake')
     self.assert_generated_output('frank', 'frank')
+    self.assert_generated_output('fra-nk', 'fra_nk')
 
   def test_names_with_caps(self):
     self.assert_generated_output('Jake', 'jake')
     self.assert_generated_output('FranK', 'frank')
 
+  def test_multiple_underscores(self):
+    self.assert_generated_output('ja__ke', 'ja_ke')
+    self.assert_generated_output('ja___ke', 'ja_ke')
+
+  def test_trailing_underscores(self):
+    self.assert_generated_output('ja__', 'ja00')
+    self.assert_generated_output('jake__', 'jake')
+
   def test_short_names(self):
-    self.assert_generated_output('a', 'a___')
-    self.assert_generated_output('ab', 'ab__')
-    self.assert_generated_output('abc', 'abc_')
+    self.assert_generated_output('a', 'a000')
+    self.assert_generated_output('ab', 'ab00')
+    self.assert_generated_output('abc', 'abc0')
 
   def test_long_names(self):
     self.assert_generated_output('abcdefghijklmnopqrstuvwxyz1234567890',
@@ -108,16 +117,18 @@ class TestUsernameGenerator(unittest.TestCase):
     self.assert_generated_output(u'\u0985\u09ad\u09bf\u099c\u09c0\u09a4', 'abhijiit')
     self.assert_generated_output(u'\u0d05\u0d2d\u0d3f\u0d1c\u0d40\u0d24', 'abhijiit')
     self.assert_generated_output(u'\u0d2e\u0d32\u0d2f\u0d3e\u0d32\u0d2e\u0d4d', 'mlyaalm')
-    self.assert_generated_output(u'\ue000', '____')
-    self.assert_generated_output(u'\u03ff', '____')
+    self.assert_generated_output(u'\ue000', '0000')
+    self.assert_generated_output(u'\u03ff', '0000')
+
+    self.assert_generated_output(u'\u0d2e\u0d32\u03ff\u03ff\u0d2e\u0d32', 'mlml')
 
   def test_multiple_suggestions(self):
     name_gen = generate_valid_usernames('a')
     generated_output = list(islice(name_gen, 4))
-    self.assertEquals('a___', generated_output[0])
-    self.assertEquals('a__0', generated_output[1])
-    self.assertEquals('a__1', generated_output[2])
-    self.assertEquals('a__2', generated_output[3])
+    self.assertEquals('a000', generated_output[0])
+    self.assertEquals('a001', generated_output[1])
+    self.assertEquals('a002', generated_output[2])
+    self.assertEquals('a003', generated_output[3])
 
 
 if __name__ == '__main__':
diff --git a/util/validation.py b/util/validation.py
index e9b954281..085fb0190 100644
--- a/util/validation.py
+++ b/util/validation.py
@@ -6,7 +6,8 @@ import anunidecode
 INVALID_PASSWORD_MESSAGE = 'Invalid password, password must be at least ' + \
                            '8 characters and contain no whitespace.'
 INVALID_USERNAME_CHARACTERS = r'[^a-z0-9_]'
-VALID_CHARACTERS = '_' + string.digits + string.lowercase
+VALID_CHARACTERS = string.digits + string.lowercase
+
 MIN_LENGTH = 4
 MAX_LENGTH = 30
 
@@ -48,8 +49,13 @@ def _gen_filler_chars(num_filler_chars):
 
 
 def generate_valid_usernames(input_username):
+  # Docker's regex: [a-z0-9]+(?:[._-][a-z0-9]+)*
   normalized = input_username.encode('unidecode', 'ignore').strip().lower()
   prefix = re.sub(INVALID_USERNAME_CHARACTERS, '_', normalized)[:30]
+  prefix = re.sub(r'_{2,}', '_', prefix)
+
+  if prefix.endswith('_'):
+    prefix = prefix[0:len(prefix) - 1]
 
   num_filler_chars = max(0, MIN_LENGTH - len(prefix))