exists函数确定是否定义了一个具有由字符串定义的名称的变量(在本例中为全局变量g： syntax_on)。

The exists function determines whether a variable with the name specified by the string (in this case, the global variable g: syntax_on) has been defined.

youdao

前向否定界定符。与肯定界定符相反；当所含表达式不能在字符串当前位置匹配时成功。

Negative lookahead assertion. This is the opposite of the positive assertion; it succeeds if the contained expression doesn't match at the current position in the string.

youdao

前向否定界定符。与肯定界定符相反；当所含表达式不能在字符串当前位置匹配时成功。

Negative lookahead assertion. This is the opposite of the positive assertion; it succeeds if the contained expression doesn't match at the current position in the string.

youdao