Java error when running JFlex generated lexer

Question

I have this basic JFlex lexer :

import java.util.*;
%%

%public
%class TuringLexer
%type Void

%init{
yybegin(YYINITIAL);
%init}

%state COMM, GETALPH, MT, PARSELOOP, PARSELEMS, PARSESYMB, PARSEMT
%{
  ArrayList<Character> alf = new ArrayList<Character>();   
  String crtMach;
  String crtLoop;
  String crtLoopContent;
  String crtLoopContentParam;
  String crtContent;
  String crtSymb;
%}

//Input = [^\r\n]
SEP = [:space:]*
//COMM =[;.*$] 
name = [A-Za-z_]*
tok=[A-Za-z0-9#$@\*]
AL = "alphabet :: "
cont = [^]]*
param =[^)]*
letter = [A-Za-z]
opn = [\[?]
symb = [^\}]+
%%
 <COMM> {
  "."  { /* ignore */  System.out.println("Got into comm state ");}
  "\n" {System.out.println("Got out of comm state ");yybegin(YYINITIAL);}
}
 <GETALPH> {
 {SEP} { /* ignore */ }
 {tok} { String str = yytext();
     System.out.println("Alphabet -- " + str);
     Character c = str.charAt(0);
     alf.add(c); }
 ";"  {yybegin(YYINITIAL);}

}
 <YYINITIAL> {
 "\n"   { /* ignore */ System.out.println("Got into YYINITIAL"); }
 ";"  { yybegin(COMM); }

[^]                    { throw new Error("Illegal character <"+yytext()+">"); }
}

Code has been removed for clarity, but the issue still persists so it is easier to identify it here.

this is the input file -> file is called simple.mt

And this is the main class :

import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.Reader;
import java.io.BufferedReader;
import java.io.FileReader;
public class MainClass  {
public static void main(String args[]) throws IOException {
    Reader reader = new BufferedReader(new FileReader ("simple.mt"));
    reader.read();
    TuringLexer tl = new TuringLexer(reader);
    tl.yylex();
}
}

When I run the project in eclipse ( or terminal, for that matter) I get:

Exception in thread "main" java.lang.Error: Illegal character <l>
    at TuringLexer.yylex(TuringLexer.java:576)
    at MainClass.main(MainClass.java:11)

I have no idea what the error means and how can I debug it, what remained from the jflex file is a small sample so the error shouldn't be that hard to figure out

Answer 1

So you have a character appearing in your input that you don't know how to handle.

All lex files should have a final . rule that either prints an 'illegal character' error message (not a thrown exception), or else just returns yytext[0] to the parser for the parser to deal with.

The latter strategy also saves you from having to write a rule for each special character, for example =, + and so on: the parser should just use them as '=', '+', etc. Then (a) any illegal character just becomes a syntax error, but more importantly (b) the parser gets to use its error recovery, rather than just throwing the token away.

Answer 2

You either do not show all the grammar or the grammar is incomplete.

Exception in thread "main" java.lang.Error: Illegal character <l>

This message tell that you don't handle loop keywords.