In ANTLR, how do you specify a specific number of repetitions? - antlr

Question

I'm using ANTLR to specify a file format that contains lines that cannot exceed 254 characters (excluding line endings). How do I encode this in the grammer, short of doing:
line : CHAR? CHAR? CHAR? CHAR? ... (254 times)

Answer 1

This can be handled by using a semantic predicate.
First write your grammar in such a way that it does not matter how long your lines are. An example would look like this:
grammar Test;
parse
: line* EOF
;
line
: Char+ (LineBreak | EOF)
| LineBreak // empty line!
;
LineBreak : '\r'? '\n' | '\r' ;
Char : ~('\r' | '\n') ;
and then add the "predicate" to the line rule:
grammar Test;
#parser::members {
public static void main(String[] args) throws Exception {
String source = "abcde\nfghij\nklm\nnopqrst";
ANTLRStringStream in = new ANTLRStringStream(source);
TestLexer lexer = new TestLexer(in);
CommonTokenStream tokens = new CommonTokenStream(lexer);
TestParser parser = new TestParser(tokens);
parser.parse();
}
}
parse
: line* EOF
;
line
: (c+=Char)+ {$c.size()<=5}? (LineBreak | EOF)
| LineBreak // empty line!
;
LineBreak : '\r'? '\n' | '\r' ;
Char : ~('\r' | '\n') ;
The c+=Char will construct an ArrayList containing all characters in the line. The {$c.size()<=5}? causes to throw an exception when the ArrayList's size exceeds 5.
I also added a main method in the parser so you can test it yourself:
// *nix/MacOSX
java -cp antlr-3.2.jar org.antlr.Tool Test.g
javac -cp antlr-3.2.jar *.java
java -cp .:antlr-3.2.jar TestParser
// Windows
java -cp antlr-3.2.jar org.antlr.Tool Test.g
javac -cp antlr-3.2.jar *.java
java -cp .;antlr-3.2.jar TestParser
which will output:
line 0:-1 rule line failed predicate: {$c.size()<=5}?
HTH