Flex/Lex : Lookahead predicate - unrecognized rule

Question

When I try to define a token like this:

NAP (([0-9])/([a-z]))

...which means a digit only if a letter come after it (r/s), and I try to use this:

{NAP}   showToken("NAP");

...the compiler throw "unrecognized rule".

Does anyone know why?

Answer 1

You cannot use flex's / operator inside parentheses. That can make it impossible to use in a definition, depending on your flex version, because flex implicitly surrounds the expansion of definitions with parentheses.

Just write your regex directly in the rule. (And no need for redundant parentheses around brackets.)

[0-9]/[a-z]     showToken("NAP");

Answer 2

It works for me.

NAP [0-9]
%%
{NAP}/([a-z])   showToken("NAP");
%%