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 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.