Flex（词法分析器）{+}和{-}运算符

Question

I am trying to make a simple scanner using Flex. In the declarations section, I am trying to use the {-} operator to exclude reserved words from an id, but I can't get it to work. Every example I have found uses the {+} and {-} operators as in the following code:

    [a-z]{-}[d]

However, I am trying to use these operators as in the following code, but I always get errors:

    invalid_id   "char"|"else"|"if"|"class"|"new"|"return"|"void"|"while"|"int"
    all_ids      [a-zA-Z_][a-zA-Z0-9_]*
    id           {all_ids}{-}{invalid_id}

Is there any way to make it work? Can these operators be used without square brackets?

Answer 1

The {-} and {+} operators only work on character classes like [az] , not on full regular expressions or strings. flex does not have support for a more general {-} operator. The more general version of {+} of course is | . Your particular problem is in general solved by the fact that if two patterns match the same string, then the first pattern will be used. So changing your specification to the following will actually exclude all keywords from the ID s.

%%
"char"|"else"|"if"|"class"|"new"|"return"|"void"|"while"|"int"  { return KEYWORD; }
[a-zA-Z_][a-zA-Z0-9_]*   { return ID; }
%%