diff --git a/python/ycm/tests/test_utils.py b/python/ycm/tests/test_utils.py
index 015856cb..5a1086f0 100644
--- a/python/ycm/tests/test_utils.py
+++ b/python/ycm/tests/test_utils.py
@@ -37,30 +37,30 @@ import sys
 from ycmd.utils import GetCurrentDirectory, ToBytes, ToUnicode
 
 
-BUFNR_REGEX = re.compile( '^bufnr\(\'(?P<buffer_filename>.+)\', ([01])\)$' )
-BUFWINNR_REGEX = re.compile( '^bufwinnr\((?P<buffer_number>[0-9]+)\)$' )
+BUFNR_REGEX = re.compile( '^bufnr\\(\'(?P<buffer_filename>.+)\', ([01])\\)$' )
+BUFWINNR_REGEX = re.compile( '^bufwinnr\\((?P<buffer_number>[0-9]+)\\)$' )
 BWIPEOUT_REGEX = re.compile(
   '^(?:silent! )bwipeout!? (?P<buffer_number>[0-9]+)$' )
 GETBUFVAR_REGEX = re.compile(
-  '^getbufvar\((?P<buffer_number>[0-9]+), "(?P<option>.+)"\)$' )
+  '^getbufvar\\((?P<buffer_number>[0-9]+), "(?P<option>.+)"\\)$' )
 MATCHADD_REGEX = re.compile(
-  '^matchadd\(\'(?P<group>.+)\', \'(?P<pattern>.+)\'\)$' )
-MATCHDELETE_REGEX = re.compile( '^matchdelete\((?P<id>\d+)\)$' )
+  '^matchadd\\(\'(?P<group>.+)\', \'(?P<pattern>.+)\'\\)$' )
+MATCHDELETE_REGEX = re.compile( '^matchdelete\\((?P<id>\\d+)\\)$' )
 OMNIFUNC_REGEX_FORMAT = (
-  '^{omnifunc_name}\((?P<findstart>[01]),[\'"](?P<base>.*)[\'"]\)$' )
-FNAMEESCAPE_REGEX = re.compile( '^fnameescape\(\'(?P<filepath>.+)\'\)$' )
+  '^{omnifunc_name}\\((?P<findstart>[01]),[\'"](?P<base>.*)[\'"]\\)$' )
+FNAMEESCAPE_REGEX = re.compile( '^fnameescape\\(\'(?P<filepath>.+)\'\\)$' )
 SIGN_LIST_REGEX = re.compile(
-  "^silent! sign place buffer=(?P<bufnr>\d+)$" )
+  '^silent! sign place buffer=(?P<bufnr>\\d+)$' )
 SIGN_PLACE_REGEX = re.compile(
-  '^sign place (?P<id>\d+) name=(?P<name>\w+) line=(?P<line>\d+) '
-  'buffer=(?P<bufnr>\d+)$' )
+  '^sign place (?P<id>\\d+) name=(?P<name>\\w+) line=(?P<line>\\d+) '
+  'buffer=(?P<bufnr>\\d+)$' )
 SIGN_UNPLACE_REGEX = re.compile(
-  '^sign unplace (?P<id>\d+) buffer=(?P<bufnr>\d+)$' )
-REDIR_START_REGEX = re.compile( '^redir => (?P<variable>[\w:]+)$' )
+  '^sign unplace (?P<id>\\d+) buffer=(?P<bufnr>\\d+)$' )
+REDIR_START_REGEX = re.compile( '^redir => (?P<variable>[\\w:]+)$' )
 REDIR_END_REGEX = re.compile( '^redir END$' )
-EXISTS_REGEX = re.compile( '^exists\( \'(?P<option>[\w:]+)\' \)$' )
-LET_REGEX = re.compile( '^let (?P<option>[\w:]+) = (?P<value>.*)$' )
-HAS_PATCH_REGEX = re.compile( '^has\( \'patch(?P<patch>\d+)\' \)$' )
+EXISTS_REGEX = re.compile( '^exists\\( \'(?P<option>[\\w:]+)\' \\)$' )
+LET_REGEX = re.compile( '^let (?P<option>[\\w:]+) = (?P<value>.*)$' )
+HAS_PATCH_REGEX = re.compile( '^has\\( \'patch(?P<patch>\\d+)\' \\)$' )
 
 # One-and only instance of mocked Vim object. The first 'import vim' that is
 # executed binds the vim module to the instance of MagicMock that is created,
diff --git a/python/ycm/tests/vimsupport_test.py b/python/ycm/tests/vimsupport_test.py
index 59803557..fcd03d88 100644
--- a/python/ycm/tests/vimsupport_test.py
+++ b/python/ycm/tests/vimsupport_test.py
@@ -1339,7 +1339,7 @@ def GetDiagnosticMatchPattern_ErrorInMiddleOfLine_test():
   with patch( 'vim.current.buffer', current_buffer ):
     assert_that(
       vimsupport.GetDiagnosticMatchPattern( 1, 16, 1, 21 ),
-      equal_to( '\%1l\%16c\_.\{-}\%1l\%21c' )
+      equal_to( '\\%1l\\%16c\\_.\\{-}\\%1l\\%21c' )
     )
 
 
@@ -1352,7 +1352,7 @@ def AddDiagnosticSyntaxMatch_WarningAtEndOfLine_test():
   with patch( 'vim.current.buffer', current_buffer ):
     assert_that(
       vimsupport.GetDiagnosticMatchPattern( 1, 16, 1, 23 ),
-      equal_to( '\%1l\%16c\_.\{-}\%1l\%23c' )
+      equal_to( '\\%1l\\%16c\\_.\\{-}\\%1l\\%23c' )
     )
 
 
@@ -1365,7 +1365,7 @@ def AddDiagnosticSyntaxMatch_UnicodeAtEndOfLine_test():
   with patch( 'vim.current.buffer', current_buffer ):
     assert_that(
       vimsupport.GetDiagnosticMatchPattern( 1, 16, 1, 19 ),
-      equal_to( '\%1l\%16c\_.\{-}\%1l\%19c' )
+      equal_to( '\\%1l\\%16c\\_.\\{-}\\%1l\\%19c' )
     )
 
 
diff --git a/python/ycm/tests/youcompleteme_test.py b/python/ycm/tests/youcompleteme_test.py
index b8067d07..71e1ad63 100644
--- a/python/ycm/tests/youcompleteme_test.py
+++ b/python/ycm/tests/youcompleteme_test.py
@@ -126,9 +126,10 @@ def RunNotifyUserIfServerCrashed( ycm, test, post_vim_message ):
 
 
 def YouCompleteMe_NotifyUserIfServerCrashed_UnexpectedCore_test():
-  message = ( "The ycmd server SHUT DOWN \(restart with ':YcmRestartServer'\). "
-              "Unexpected error while loading the YCM core library. Type "
-              "':YcmToggleLogs ycmd_\d+_stderr_.+.log' to check the logs." )
+  message = (
+    "The ycmd server SHUT DOWN \\(restart with ':YcmRestartServer'\\). "
+    "Unexpected error while loading the YCM core library. Type "
+    "':YcmToggleLogs ycmd_\\d+_stderr_.+.log' to check the logs." )
   RunNotifyUserIfServerCrashed( {
     'return_code': 3,
     'expected_message': matches_regexp( message )
@@ -178,9 +179,10 @@ def YouCompleteMe_NotifyUserIfServerCrashed_OutdatedCore_test():
 
 
 def YouCompleteMe_NotifyUserIfServerCrashed_UnexpectedExitCode_test():
-  message = ( "The ycmd server SHUT DOWN \(restart with ':YcmRestartServer'\). "
-              "Unexpected exit code 1. Type "
-              "':YcmToggleLogs ycmd_\d+_stderr_.+.log' to check the logs." )
+  message = (
+    "The ycmd server SHUT DOWN \\(restart with ':YcmRestartServer'\\). "
+    "Unexpected exit code 1. Type "
+    "':YcmToggleLogs ycmd_\\d+_stderr_.+.log' to check the logs." )
   RunNotifyUserIfServerCrashed( {
     'return_code': 1,
     'expected_message': matches_regexp( message )
@@ -212,7 +214,7 @@ def YouCompleteMe_DebugInfo_ServerRunning_test( ycm ):
         '  Flags: \\[u?\'_TEMP_FILE_\'.*\\]\n'
         '  Translation unit: .+\n)'
         'Server running at: .+\n'
-        'Server process ID: \d+\n'
+        'Server process ID: \\d+\n'
         'Server logfiles:\n'
         '  .+\n'
         '  .+' )
@@ -231,7 +233,7 @@ def YouCompleteMe_DebugInfo_ServerNotRunning_test( ycm ):
         'Client logfile: .+\n'
         'Server errored, no debug info from server\n'
         'Server running at: .+\n'
-        'Server process ID: \d+\n'
+        'Server process ID: \\d+\n'
         'Server logfiles:\n'
         '  .+\n'
         '  .+' )
@@ -626,9 +628,9 @@ def YouCompleteMe_UpdateDiagnosticInterface_PrioritizeErrorsOverWarnings_test(
       test_utils.VIM_MATCHES_FOR_WINDOW,
       has_entries( {
         1: contains(
-          VimMatch( 'YcmWarningSection', '\%3l\%5c\_.\{-}\%3l\%7c' ),
-          VimMatch( 'YcmWarningSection', '\%3l\%3c\_.\{-}\%3l\%9c' ),
-          VimMatch( 'YcmErrorSection', '\%3l\%8c' )
+          VimMatch( 'YcmWarningSection', '\\%3l\\%5c\\_.\\{-}\\%3l\\%7c' ),
+          VimMatch( 'YcmWarningSection', '\\%3l\\%3c\\_.\\{-}\\%3l\\%9c' ),
+          VimMatch( 'YcmErrorSection', '\\%3l\\%8c' )
         )
       } )
     )
@@ -670,8 +672,8 @@ def YouCompleteMe_UpdateDiagnosticInterface_PrioritizeErrorsOverWarnings_test(
       test_utils.VIM_MATCHES_FOR_WINDOW,
       has_entries( {
         1: contains(
-          VimMatch( 'YcmWarningSection', '\%3l\%5c\_.\{-}\%3l\%7c' ),
-          VimMatch( 'YcmWarningSection', '\%3l\%3c\_.\{-}\%3l\%9c' )
+          VimMatch( 'YcmWarningSection', '\\%3l\\%5c\\_.\\{-}\\%3l\\%7c' ),
+          VimMatch( 'YcmWarningSection', '\\%3l\\%3c\\_.\\{-}\\%3l\\%9c' )
         )
       } )
     )
@@ -692,9 +694,9 @@ def YouCompleteMe_UpdateMatches_ClearDiagnosticMatchesInNewBuffer_test( ycm ):
 
   test_utils.VIM_MATCHES_FOR_WINDOW.clear()
   test_utils.VIM_MATCHES_FOR_WINDOW[ 1 ] = [
-    VimMatch( 'YcmWarningSection', '\%3l\%5c\_.\{-}\%3l\%7c' ),
-    VimMatch( 'YcmWarningSection', '\%3l\%3c\_.\{-}\%3l\%9c' ),
-    VimMatch( 'YcmErrorSection', '\%3l\%8c' )
+    VimMatch( 'YcmWarningSection', '\\%3l\\%5c\\_.\\{-}\\%3l\\%7c' ),
+    VimMatch( 'YcmWarningSection', '\\%3l\\%3c\\_.\\{-}\\%3l\\%9c' ),
+    VimMatch( 'YcmErrorSection', '\\%3l\\%8c' )
   ]
 
   with MockVimBuffers( [ current_buffer ], [ current_buffer ] ):
@@ -862,7 +864,7 @@ def YouCompleteMe_AsyncDiagnosticUpdate_SingleFile_test( ycm,
     test_utils.VIM_MATCHES_FOR_WINDOW,
     has_entries( {
       1: contains(
-        VimMatch( 'YcmErrorSection', '\%1l\%1c\_.\{-}\%1l\%1c' )
+        VimMatch( 'YcmErrorSection', '\\%1l\\%1c\\_.\\{-}\\%1l\\%1c' )
       )
     } )
   )
@@ -1049,10 +1051,10 @@ def YouCompleteMe_AsyncDiagnosticUpdate_PerFile_test( ycm,
     test_utils.VIM_MATCHES_FOR_WINDOW,
     has_entries( {
       1: contains(
-        VimMatch( 'YcmErrorSection', '\%1l\%1c\_.\{-}\%1l\%1c' )
+        VimMatch( 'YcmErrorSection', '\\%1l\\%1c\\_.\\{-}\\%1l\\%1c' )
       ),
       3: contains(
-        VimMatch( 'YcmErrorSection', '\%3l\%3c\_.\{-}\%3l\%3c' )
+        VimMatch( 'YcmErrorSection', '\\%3l\\%3c\\_.\\{-}\\%3l\\%3c' )
       )
     } )
   )
diff --git a/python/ycm/vimsupport.py b/python/ycm/vimsupport.py
index 088752ac..03c0f7f4 100644
--- a/python/ycm/vimsupport.py
+++ b/python/ycm/vimsupport.py
@@ -257,16 +257,16 @@ def GetDiagnosticMatchPattern( line_num,
   line_num, column_num = LineAndColumnNumbersClamped( line_num, column_num )
 
   if not line_end_num or not column_end_num:
-    return '\%{}l\%{}c'.format( line_num, column_num )
+    return '\\%{}l\\%{}c'.format( line_num, column_num )
 
   # -1 and then +1 to account for column end not included in the range.
   line_end_num, column_end_num = LineAndColumnNumbersClamped(
       line_end_num, column_end_num - 1 )
   column_end_num += 1
-  return '\%{}l\%{}c\_.\\{{-}}\%{}l\%{}c'.format( line_num,
-                                                  column_num,
-                                                  line_end_num,
-                                                  column_end_num )
+  return '\\%{}l\\%{}c\\_.\\{{-}}\\%{}l\\%{}c'.format( line_num,
+                                                       column_num,
+                                                       line_end_num,
+                                                       column_end_num )
 
 
 # Clamps the line and column numbers so that they are not past the contents of
@@ -993,13 +993,13 @@ def InsertNamespace( namespace ):
       vim.eval( expr )
       return
 
-  pattern = '^\s*using\(\s\+[a-zA-Z0-9]\+\s\+=\)\?\s\+[a-zA-Z0-9.]\+\s*;\s*'
+  pattern = r'^\s*using\(\s\+[a-zA-Z0-9]\+\s\+=\)\?\s\+[a-zA-Z0-9.]\+\s*;\s*'
   existing_indent = ''
   line = SearchInCurrentBuffer( pattern )
   if line:
     existing_line = LineTextInCurrentBuffer( line )
-    existing_indent = re.sub( r"\S.*", "", existing_line )
-  new_line = "{0}using {1};\n".format( existing_indent, namespace )
+    existing_indent = re.sub( r'\S.*', '', existing_line )
+  new_line = '{0}using {1};\n'.format( existing_indent, namespace )
   replace_pos = { 'line_num': line + 1, 'column_num': 1 }
   ReplaceChunk( replace_pos, replace_pos, new_line, vim.current.buffer )
   PostVimMessage( 'Add namespace: {0}'.format( namespace ), warning = False )
diff --git a/tox.ini b/tox.ini
index 763a9923..be38833b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,4 +1,4 @@
 [flake8]
-ignore = E111,E114,E121,E125,E126,E127,E128,E129,E131,E133,E201,E202,E203,E221,E222,E241,E251,E261,E303,E402,W503
+ignore = E111,E114,E121,E125,E126,E127,E128,E129,E131,E133,E201,E202,E203,E221,E222,E241,E251,E261,E301,E303,E402,W503,W504
 max-complexity = 10
 max-line-length = 80