This is cool. I use regular expressions a lot but can always use some visual guidance. I noticed that the flow chart you generate describing the regular expression is typically cutoff (see https://app.usebubbles.com/67a2502b-44b5-4744-be51-f2b38d8703e9) yet it's the most useful feature to me. Would you consider making that use more real estate?
Even if you don't actually hate regex this can be really useful. Especially the visual representation can help understanding what is really happening in that weird two-liner expression you wrote three years ago. Also there's several examples that can help you get started.
Good idea for improvement would be some kind of drag and drop UI for making new expressions. Keep it going :)