BNF语法定义疑问：如何用BNF描述字符串字面量赋值语句

Let's build a precise BNF that covers your example If Temperature <=0 Then Description = "Freezing" End If, including the string-to-identifier assignment that's stumping you. We'll start with the full BNF, then break down each component to clarify the assignment logic.

Complete BNF #

<program> ::= <if-statement>

<if-statement> ::= "If" <relational-expression> "Then" <statement> "End If"

<statement> ::= <assignment-statement>
               | <if-statement>  // For nested Ifs, optional but common in VBA

<assignment-statement> ::= <identifier> "=" <expression>

<expression> ::= <numeric-literal>
               | <string-literal>
               | <identifier>

<relational-expression> ::= <identifier> <relational-operator> <numeric-literal>

<relational-operator> ::= "<=" | "<" | ">=" | ">" | "=" | "<>"

<identifier> ::= <letter> (<letter> | <digit>)*

<numeric-literal> ::= <digit>+ ("." <digit>*)?  // Covers integers and decimals, e.g., 0, 12.34

<string-literal> ::= """ <character>* """  // VBA uses double quotes for strings; <character> excludes unescaped "

Key Explanation for the Assignment Statement #

The confusion around assigning a string literal to an identifier comes down to modeling the right-hand side of the assignment as an expression that can accept different literal types. Here's how it works:

• <assignment-statement> ties an <identifier> (like Description) to any valid <expression>.
• <expression> explicitly includes <string-literal> (like "Freezing") as a valid option, alongside numeric literals and other identifiers. This matches VBA's behavior where you can assign strings, numbers, or variable values to identifiers.

Mapping Your Example to the BNF #

Let's walk through how If Temperature <=0 Then Description = "Freezing" End If fits:

1. <if-statement> starts with "If"
2. <relational-expression> is Temperature (identifier) + <= (operator) + 0 (numeric literal)
3. "Then" leads to <statement>, which is an <assignment-statement>:
   • <identifier> = Description
   • "=" is the assignment operator
   • <expression> = "Freezing" (string literal)
4. Ends with "End If"

Notes for VBA Specifics #

• In full VBA, <expression> would be more complex (supporting arithmetic, function calls, etc.), but this BNF focuses on your specific example.
• For <string-literal>, VBA allows escaping double quotes by doubling them (e.g., "He said ""Hello"""), so you could extend that rule to <string-literal> ::= """ (<character> | """)* """ if needed.

内容的提问来源于stack exchange，提问作者Drummy