From ChatGPT we can learn:
In programming, "alphabet" generally refers to the set of characters that can be used to write programs in a particular programming language. This includes letters (both uppercase and lowercase), digits, punctuation marks, and special symbols that are recognized by the language's syntax. The alphabet defines what characters are valid for variable names, keywords, operators, and other elements of the language's syntax. OpenAI. (2024). *ChatGPT* (3.5) [Large language model].
I agree 100% without comments. The question is how to define this set to allow defining syntax rules atop it.