ANTLR: how to parse a region within matching brackets with a lexer - antlr

Question

i want to parse something like this in my lexer:
( begin expression )
where expressions are also surrounded by brackets. it isn't important what is in the expression, i just want to have all what's between the (begin and the matching ) as a token. an example would be:
(begin
(define x (+ 1 2)))
so the text of the token should be (define x (+ 1 2)))
something like
PROGRAM : LPAREN BEGIN .* RPAREN;
does (obviously) not work because as soon as he sees a ")", he thinks the rule is over, but i need the matching bracket for this.
how can i do that?

Answer 1

Inside lexer rules, you can invoke rules recursively. So, that's one way to solve this. Another approach would be to keep track of the number of open- and close parenthesis and let a gated semantic predicate loop as long as your counter is more than zero.
A demo:
T.g
grammar T;
parse
: BeginToken {System.out.println("parsed :: " + $BeginToken.text);} EOF
;
BeginToken
#init{int open = 1;}
: '(' 'begin' ( {open > 0}?=> // keep reapeating `( ... )*` as long as open > 0
( ~('(' | ')') // match anything other than parenthesis
| '(' {open++;} // match a '(' in increase the var `open`
| ')' {open--;} // match a ')' in decrease the var `open`
)
)*
;
Main.java
import org.antlr.runtime.*;
public class Main {
public static void main(String[] args) throws Exception {
String input = "(begin (define x (+ (- 1 3) 2)))";
TLexer lexer = new TLexer(new ANTLRStringStream(input));
TParser parser = new TParser(new CommonTokenStream(lexer));
parser.parse();
}
}
java -cp antlr-3.3-complete.jar org.antlr.Tool T.g
javac -cp antlr-3.3-complete.jar *.java
java -cp .:antlr-3.3-complete.jar Main
parsed :: (begin (define x (+ (- 1 3) 2)))
Note that you'll need to beware of string literals inside your source that might include parenthesis:
BeginToken
#init{int open = 1;}
: '(' 'begin' ( {open > 0}?=> // ...
( ~('(' | ')' | '"') // ...
| '(' {open++;} // ...
| ')' {open--;} // ...
| '"' ... // TODO: define a string literal here
)
)*
;
or comments that may contain parenthesis.
The suggestion with the predicate uses some language specific code (Java, in this case). An advantage of calling a lexer rule recursively is that you don't have custom code in your lexer:
BeginToken
: '(' Spaces? 'begin' Spaces? NestedParens Spaces? ')'
;
fragment NestedParens
: '(' ( ~('(' | ')') | NestedParens )* ')'
;
fragment Spaces
: (' ' | '\t')+
;