In the Regular Expressions HOWTO (https://docs.python.org/3/howto/regex.html#performing-matches, Doc/howto/regex.rst), there is the following paragraph:

=====
You can learn about this by interactively experimenting with the :mod:`re`
module.  If you have :mod:`tkinter` available, you may also want to look at
:source:`Tools/demo/redemo.py`, a demonstration program included with the
Python distribution.  It allows you to enter REs and strings, and displays
whether the RE matches or fails. :file:`redemo.py` can be quite useful when
trying to debug a complicated RE.  Phil Schwartz's `Kodos
<http://kodos.sourceforge.net/>`_ is also an interactive tool for developing and
testing RE patterns.
=====

I checked the Kodos site on SourceForge, and the latest release is exactly 10 years old. I couldn't get it to work with 2.7.11 on Ubuntu 16.04 and PyQt4 - Qt3 is one of its requirements. I didn't bother running 2to3 on it. I propose we delete the last sentence of the above paragraph altogether. There are many good regex sites out there, including regex101, regexr, Debuggex, regexpal, and more (those are just the first few in my bookmarks list), and I don't feel we should be promoting one outdated tool.

Here is the diff:
=====
@@ -374,9 +374,7 @@
 :source:`Tools/demo/redemo.py`, a demonstration program included with the
 Python distribution.  It allows you to enter REs and strings, and displays
 whether the RE matches or fails. :file:`redemo.py` can be quite useful when
-trying to debug a complicated RE.  Phil Schwartz's `Kodos
-<http://kodos.sourceforge.net/>`_ is also an interactive tool for developing and
-testing RE patterns.
+trying to debug a complicated RE.

 This HOWTO uses the standard Python interpreter for its examples. First, run the
 Python interpreter, import the :mod:`re` module, and compile a RE::

=====

I've also attached it as a file.

New changeset 23182404a85c by Berker Peksag in branch '3.5':
Issue #27378: Remove an outdated reference from regex HOWTO
https://hg.python.org/cpython/rev/23182404a85c

New changeset 21cb299a9626 by Berker Peksag in branch 'default':
Issue #27378: Merge from 3.5
https://hg.python.org/cpython/rev/21cb299a9626

Thanks!