I have just been shown this is a really cool tool (thanks Pete). Enter a regular expression into the box at the top and it will generate a visual flow diagram of what the regular expression is doing. I think this the presentation is spot on.
The site is at https://regexper.com/
Shown below, I picked a random regular expression from the regular expression library, http://regexlib.com/ and then pasted it into the tool to see what it looked like.
I can see the application of this too, for quickly orientating yourself around what a regular expression written a long time ago, the intent now lost, was doing or perhaps a new one like this that has been picked up from the internet.
I can see that the tool could also be useful for finding those silly mistakes when developing expressions and finding they don’t quite do what you expected, as it will provide another view on the problem.