LookAhead无法在JFlex中运行

Question

I am trying to use JFlex to build a parser but encounter a very basic issue. I want to have this token

[A-Za-z]+_N$ { System.out.println("Noun"); }

and have it recognize "car_N" as a "Noun" and NOT recognize "car_NN". but it does not work, if I use

[A-Za-z]+_N { System.out.println("Noun"); } 

"car_N" get recognize but "car_NN" returns "car_N" as well, this is not what I want.

I wonder anyone here knows how to help?

JFlex and Flex lookahead symbol '$' does not seem to be working

Answer 1

I think you're misunderstanding the meaning of '$'. As a lookahead, it matches only at the end of a line of text (hard-line break). As such it works perfectly fine. But it cannot differentiate between car_N and car_NN, except for instances of car_N that are at the very end of a line. Maybe you really want a lookahead something like:

[A-Za-z]+_N/[^A-Za-z]