如何在Flex中定义布尔值（2）

Question

I am writing a scanner for my compiler project using flex and writing it in decaf (a language similar to C/C++ & Java), my scanner is up and running, except it fails in identifying boolean constants (true, false) I wrote below pattern and C code:

"true" { yylval.boolConstant = 1;
       return T_BoolConstant; }

"false" { yylval.boolConstant = 0;
        return T_BoolConstant; }

but instead of returning T_BoolConstant it keeps returning as T_Identifier . Basically scanner is recognizing both "true" & "false" as text valid to be considered as identifier not as boolean binary variable.

There is this answer for similar type of question but still it doesn't work for me!

Any guess why it isn't returning true/false ?

Answer 1

This will be because you have your rule for identifiers above these rules. Flex matches rules top to bottom.