The CLP recognizes' null '(specified in uppercase characters) as a null string. For example.

CLP将“NULL”(以大写字符指定)识别为空字符串。

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It returns a null if no match is found, or if the source string is null or empty.

如果没有发现匹配项或源字符串是null或空，则返回null。

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Both null and undefined macros expand to null strings, but a macro defined as a null string is considered defined in preprocessing expressions.

空宏和未定义的宏都展开为空字符串，但定义为空字符串的宏被视为是在预处理表达式中定义的。

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Both null and undefined macros expand to null strings, but a macro defined as a null string is considered defined in preprocessing expressions.

空宏和未定义的宏都展开为空字符串，但定义为空字符串的宏被视为是在预处理表达式中定义的。

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