I saw in the plantuml repository that a description for EBNF existed and that there was now a reference from README.md to it (pull request #1275 thanks @The-Lu).
In the plantuml code I saw also that there are possibilities for:
- regex, probably related to the question https://forum.plantuml.net/17112/regex-railroad-diagrams?show=17112#q17112,
- hcl, no idea what it is / where hcl stands for.
Are there for this: