Question

I’m trying to write a program in ANTLR (Java) concerning simplifying regular expression. I have already written some code (grammar file contents below)

grammar Regexp_v7;
options{
    language = Java;
    output = AST;
    ASTLabelType = CommonTree;
    backtrack = true;
}

tokens{
    DOT;
    REPEAT;
    RANGE;
    NULL;
} 

fragment
    ZERO
            :    '0'
            ;

fragment
    DIGIT
            :    '1'..'9'
            ;

fragment
    EPSILON
            :    '@'
            ;

fragment
    FI
            :    '%'
            ;

    ID
            :    EPSILON
            |    FI
            |    'a'..'z'
            |    'A'..'Z'
            ;

 NUMBER
            :    ZERO
            |    DIGIT (ZERO | DIGIT)*
            ;

 WHITESPACE
            :    ('\r' | '\n' | ' ' | '\t' ) + {$channel = HIDDEN;}
            ;

list
            :    (reg_exp ';'!)*
            ;

term
            :        ID -> ID
            |    '('! reg_exp ')'!
            ;

repeat_exp
            :    term ('{' range_exp '}')+ -> ^(REPEAT term (range_exp)+)
            |    term -> term
            ;

range_exp
            :    NUMBER ',' NUMBER -> ^(RANGE NUMBER NUMBER)
            |    NUMBER (',') -> ^(RANGE NUMBER NULL)
            |    ',' NUMBER -> ^(RANGE NULL NUMBER)
            |    NUMBER -> ^(RANGE NUMBER NUMBER)
            ;
kleene_exp
            :    repeat_exp ('*'^)*
            ;
concat_exp
            :    kleene_exp (kleene_exp)+ -> ^(DOT kleene_exp (kleene_exp)+)
            |    kleene_exp -> kleene_exp
            ;

reg_exp
            :    concat_exp ('|'^ concat_exp)*
            ;

My next goal is to write down tree grammar code, which is able to simplify regular expressions (e.g. a|a -> a , etc.). I have done some coding (see text below), but I have troubles with defining rule that treats nodes as subtrees (in order to simplify following kind of expressions e.g.: (a|a)|(a|a) to a, etc.)

tree grammar Regexp_v7Walker;

options{
    language = Java;
    tokenVocab = Regexp_v7;
    ASTLabelType = CommonTree;
    output=AST;
    backtrack = true;
}

tokens{
    NULL;
}

bottomup
            : ^('*' ^('*' e=.)) -> ^('*' $e)    //a** -> a*
            | ^('|' i=.* j=.* {$i.tree.toStringTree() == $j.tree.toStringTree()} ) 
            -> $i // There are 3 errors while this line is up and running: 
                  // 1. CommonTree cannot be resolved, 
                  // 2. i.tree cannot be resolved or is not a field,
                  // 3. i cannot be resolved.
;

Small driver class:

public class Regexp_Test_v7 {
    public static void main(String[] args) throws RecognitionException {
        CharStream stream = new ANTLRStringStream("a***;a|a;(ab)****;ab|ab;ab|aa;");
        Regexp_v7Lexer lexer = new Regexp_v7Lexer(stream);
        CommonTokenStream tokenStream = new CommonTokenStream(lexer);
        Regexp_v7Parser parser = new Regexp_v7Parser(tokenStream);
        list_return list = parser.list();
        CommonTree t = (CommonTree) list.getTree();
        System.out.println("Original tree: " + t.toStringTree());
        CommonTreeNodeStream nodes = new CommonTreeNodeStream(t);
        Regexp_v7Walker s = new Regexp_v7Walker(nodes);
        t = (CommonTree)s.downup(t);
        System.out.println("Simplified tree: " + t.toStringTree());

Can anyone help me with solving this case?
Thanks in advance and regards.

Answer 1

Now, I’m no expert, but in your tree grammar:

1. add filter=true
2. change the second line of bottomup rule to:
   ^('|' i=. j=. {i.toStringTree().equals(j.toStringTree()) }? ) -> $i }

If I’m not mistaken by using i=.* you’re allowing i to be non-existent and you’ll get a NullPointerException on conversion to a String.

Both i and j are of type CommonTree because you’ve set it up this way: ASTLabelType = CommonTree, so you should call i.toStringTree().

And since it’s Java and you’re comparing Strings, use equals().

Also to make the expression in curly brackets a predicate, you need a question mark after the closing one.