How to match something not defined

Question

if I have defined something like that

COMMAND         = "HI" | "HOW" | "ARE" | "YOU"

How can i say "if u match something that is not a COMMAND"?..

I tried with this one

[^COMMAND]

But didn't work..

But in that case u can match only a word.. if you write ^((?!BYE|HI).)*$ using OR operator doesn't work.. — Vittorio Cortosi, May 31 '18 at 19:56
Im using Jflex.. it notice me like a syntax error.. can it be written differently? — Vittorio Cortosi, May 31 '18 at 20:55

score 0 · Accepted Answer · answered May 31 '18 at 22:58

As far as I can tell this is not possible with (current) JFlex.

We would need an effective tempered negative lookahead: ((?!bad).)*

There are two ways to do a negative lookahead in JFlex:

negation in the lookahead: x / !(y [^]*) (match x if not followed by y in the lookahead).
lookahead with negated elements: x / [^y]|y[^z] (match if x is followed by something that is !a or a!b.

Otherwise, you may get some ideas from this answer (specifically the lookaround alternatives): https://stackoverflow.com/a/37988661/8291949

score 0 · Answer 2 · answered Sep 13 '18 at 16:58

0

Well, you can just match anything else, then

COMMAND         = "HI" | "HOW" | "ARE" | "YOU" 
. {throw new RuntimeException("Illegal character: <" + yytext() + ">");}

answered Sep 13 '18 at 16:58

rds

2 Answers2