This fairly common piece of software does it.

https://www.regexbuddy.com

4th result on google for "regex conversion" for me. Seems like there are quite a few others.

I'm familiar with that program, but I see no railroad diagrams there.

They're replying to the "convert" part, which regexbuddy does [1]

[1] https://www.regexbuddy.com/convert.html

You seem to be chasing the idea that somehow your idea was formative, even though pretty much nobody will have seen it, even though you yourself don't know whether or not there were earlier instances.

Railroad diagrams are also called syntax diagrams [1] and were used in print as early as 1973 [2]. They are an obvious kind of diagram to use for regular expressions, which express a grammar/syntax.

Basically, many people will have had the idea, independently, because representing a regex as a railroad diagram is an obvious invention, as is converting them between variants. Doing these on demand is also obvious.

Trying to take some sort of credit (even as inspiration) for other people's work is not likely to make you many friends. The world is a vast place with many things going on independently. Keep on inventing.

[1] https://en.wikipedia.org/wiki/Syntax_diagram [2] Niklaus Wirth: The Programming Language Pascal. (July 1973)

I think the first place I such a diagram was in the Smalltalk blue book. It has railroad diagrams for the language grammar. So not regular expressions, but very similar.