What are "regular-char" and "regular-string-char" in the doc?

Learning
1

https://v2.ocaml.org/manual/lex.html#string-character

string-character ::= regular-string-char | …
char-literal ::= ’ regular-char ’ | …

2

Anything that is neither an escape nor a quote (resp. double quote) nor a line break.

3

What is “anything”? A byte? A utf8 thing? Other?

4

Any byte, and an utf8-valid byte is a byte.