eliminate extra spaces in a given ANTLR grammar - antlr

Question

In any grammar I create in ANTLR, is it possible to parse the grammar and the result of the parsing can eleminate any extra spaces in the grammar. f.e
simple example ;
int x=5;
if I write
int x = 5 ;
I would like that the text changes to the int x=5 without the extra spaces. Can the parser return the original text without extra spaces?

Answer 1

Can the parser return the original text without extra spaces?
Yes, you need to define a lexer rule that captures these spaces and then skip() them:
Space
: (' ' | '\t') {skip();}
;
which will cause spaces and tabs to be ignored.
PS. I'm assuming you're using Java as the target language. The skip() can be different in other targets (Skip() for C#, for example). You may also want to include \r and \n chars in this rule.
EDIT
Let's say your language only consists of a couple of variable declarations. Assuming you know the basics of ANTLR, the following grammar should be easy to understand:
grammar T;
parse
: stat* EOF
;
stat
: Type Identifier '=' Int ';'
;
Type
: 'int'
| 'double'
| 'boolean'
;
Identifier
: ('a'..'z' | 'A'..'Z' | '_') ('a'..'z' | 'A'..'Z' | '_' | '0'..'9')*
;
Int
: '0'..'9'+
;
Space
: (' ' | '\t' | '\n' | 'r')+ {skip();}
;
And you're parsing the source:
int x = 5 ; double y =5;boolean z = 0 ;
which you'd like to change into:
int x=5;
double y=5;
boolean z=0;
Here's a way to embed code in your grammar and let the parser rules return custom objects (Strings, in this case):
grammar T;
parse returns [String str]
#init{StringBuilder buffer = new StringBuilder();}
#after{$str = buffer.toString();}
: (stat {buffer.append($stat.str).append('\n');})* EOF
;
stat returns [String str]
: Type Identifier '=' Int ';'
{$str = $Type.text + " " + $Identifier.text + "=" + $Int.text + ";";}
;
Type
: 'int'
| 'double'
| 'boolean'
;
Identifier
: ('a'..'z' | 'A'..'Z' | '_') ('a'..'z' | 'A'..'Z' | '_' | '0'..'9')*
;
Int
: '0'..'9'+
;
Space
: (' ' | '\t' | '\n' | 'r')+ {skip();}
;
Test it with the following class:
import org.antlr.runtime.*;
public class Main {
public static void main(String[] args) throws Exception {
String source = "int x = 5 ; double y =5;boolean z = 0 ;";
ANTLRStringStream in = new ANTLRStringStream(source);
TLexer lexer = new TLexer(in);
CommonTokenStream tokens = new CommonTokenStream(lexer);
TParser parser = new TParser(tokens);
System.out.println("Result:\n"+parser.parse());
}
}
which produces:
Result:
int x=5;
double y=5;
boolean z=0;