RegExp Tester App:
It should be easy to add a link to a help page about regular expressions.
And maybe right in the window one could have a listing of all the special characters
which need to be escaped.
(Pressing F1 when in the app opens the help page for apps.)
There is already a link at the bottom. Probably you need to make the app window a bit larger:
(https://www.photools.com/community/index.php?action=dlattach;topic=8296.0;attach=19840;image)