This is a PyQt5-based GUI tool to assist in the development of regular expressions. It features on-the-fly highlighting of matched text and capture groups in the text (the whole match will have red text, capture group 1 has yellow background, and so on).
A click on the items in the tree view will select the corresponding text.
It looks like this:
redemo.py as included in CPython in Tools/demo, which in Python 3.6 has a Bug where the regex flags cannot be set.