I want to know what is the name of such type of diagrams? Searching for text syntax diagrams yields only railroad diagrams, but that is different type, not what I look for.
I'm not sure there is a specific word for them; the general term is "diagramming", as you already know. I personally would lean towards calling them "anatomies", as they break down the whole into named parts.– Dan BronApr 23, 2021 at 19:38
2It's very close to what's called a "comb field" in paper form design, but really there isn't a great deal of complexity here - it's just a stylised form of bracket or curly brace such as depicted in the following link, and is a centuries-old device: macrotypography.blogspot.com/2015/11/curly-braces.html?m=1– SteveApr 23, 2021 at 20:55
They are often called decoders. Chevy VIN decoder– Phil SweetApr 29, 2021 at 20:45
I would say this is a very simple form of a "Parsing diagram".
"Parsing, syntax analysis, or syntactic analysis is the process of analyzing a string of symbols, either in natural language, computer languages or data structures, conforming to the rules of a formal grammar."
Your diagram shows an expression (an e-mail address), takes its parts, and shows how it conforms to the addressing rules.
Googling for "parsing diagram", both for All and for Images, will give you examples.