Question

In the following code that skips comments, what’s meaning of BEGIN(INITIAL) ?

    %x C_COMMENT

    "/*" { BEGIN(C_COMMENT); }
    "*/" { BEGIN(INITIAL); }
    .  { }

Answer 1

INITIAL is a state which is implicitly declared in all lex programs. (C_COMMENT is also a state, but it is not built-in into lex, so it is declared explicitly.)

BEGIN(statename) just means enter the state statename. So what this lex snippet does is the following: If an “/*” is read it enters the state C_COMMENT, when it sees a “*/” it goes back to the default state.

You could now add rules which ignore all input (except “*/” of course) when in the C_COMMENT state, while doing other things with the input when you are not (or when you are in the INITIAL state).