filter option is not in ANTLR4

1
votes

I write options{filter=true;} in lexer grammar file and compile it with ANTLR4. It said 

 unsupported option 'filter'

It seems 'filter' only works with ANTLR3

I checked wiki http://www.antlr.org/wiki/display/ANTLR4/Options and can not find the answer. 'filter' key word is not in the source file https://github.com/antlr/antlr4/blob/master/tool/src/org/antlr/v4/tool/Grammar.java either.

How could I fix this？

1
antlr4

1 Answers

0
votes

There is no filter option in ANTLR 4 lexer grammars. However, it is easy to mimic this behavior as follows:

lexer grammar L;

RULE
 : [a-zA-Z]+
 ;

FILTER
 : . -> skip
 ;

which is equivalent to the ANTLR 3 lexer grammar:

lexer grammar L;

options {
  filter=true;
}

RULE
 : ('a'..'z' | 'A'..'Z')+
 ;