计算理论与自动机技术问询：编程语言中声明的变量属于终结符（terminal symbols）还是非终结符（non-terminal symbols）

Great question—this is a common point of confusion when bridging formal language theory and real-world programming languages. Let’s break it down clearly, because the answer depends entirely on whether you’re talking about specific variable names in code or the abstract syntactic category of "variable" in a grammar.

1. Specific variable names (e.g., x, user_score, total) are terminal symbols #

Terminals are the lowest-level, indivisible tokens that a parser directly encounters in source code—they’re the "atoms" of syntax. When you write let age = 30;, the age here is a terminal: it has no substructure, and the parser won’t split it into smaller syntactic units. Think of terminals as the concrete bits you type: literals (30, "hello"), operators (+, =), and yes, specific identifiers like variable names all fall into this category.

2. The syntactic category "variable" (as a grammar rule) is a non-terminal symbol #

In a context-free grammar (CFG) for a programming language, non-terminals represent abstract syntactic categories that can be expanded into other symbols (either terminals or other non-terminals). For example, you might have grammar rules like this:

<Expression> → <Variable> | <NumericLiteral> | <Expression> + <Expression>
<Variable> → <Identifier>
<Identifier> → [a-zA-Z_][a-zA-Z0-9_]*

Here, <Variable> is a non-terminal. It’s an abstract concept that gets expanded into an <Identifier>, which in turn expands into a concrete variable name (a terminal). Non-terminals are all about describing structure, not the actual tokens present in code.

Quick Rule of Thumb #

• If you’re pointing to something you can type directly in code (like myVar): Terminal
• If you’re talking about the grammar’s way of grouping things that count as variables: Non-terminal

It’s similar to English grammar: "cat" is a terminal (a specific word), while "noun" is a non-terminal (the abstract category that includes "cat", "dog", etc.). Variables follow the same logic!

内容的提问来源于stack exchange，提问作者GAMEOVER