Java petitparser optional end of expression

Question

I need a parser that took only important marked parts from a text file. This is sample input:

else before 1
else before 2
--Start Query 1
important 1
--End 1
else between 1 and 2 - 1
else between 1 and 2 - 2
--Start Query 2
important 2
--End 2 
else after1-1
else after1-2

I wrote this parser:

public class ExpressionDefinition extends GrammarDefinition {
        {
            def("start", ref("expr").star().end());
            def("nl",  of("\r\n").or(of("\n").or(of("\r"))));
            def("expr", 
                    ref("else").starLazy(ref("expr_start").flatten())
                    .seq(ref("expr_start"))
                    .seq(ref("expr_body"))
                    .seq(ref("expr_end"))
                    .seq(ref("else").starLazy(ref("expr_start")).optional()).map(in -> {
                        if (in instanceof List) {
                            for (Object o: (List)in) {
                                if (o instanceof Body) {
                                    return o;
                                } 
                            }
                        }
                        return null;
                    }));
            def("expr_start", of("--Start Query").seq(any().starLazy(ref("nl")), ref("nl")));
            def("expr_body", any().starLazy(ref("expr_end")).flatten().map((String in) -> new Body(in)));
            def("expr_end", of("--End").seq(any().starLazy(ref("nl")).optional(), ref("nl").optional()));
            def("else", any().starLazy(ref("nl")).seq(ref("nl")));
        }

With this small utility Pojo for getting important data:

@Data
@AllArgsConstructor
public static class Body {
    private final String val;
    @Override public String toString() { return val; }
}

Run like:

ExpressionDefinition def = new ExpressionDefinition();
Parser parser = def.build();
Result result = parser.parse(input);

And it throw me an exception: org.petitparser.context.ParseError: end of input expected But no visible reason why, as the last line is else kind of content and we expecting to have it with star condition, and its a part of expr: ref("else").starLazy(ref("expr_start")).optional()

How can I change the parser, so it will expect et the end of each expr to be any amount of else with and without possible new-line character at the end of the input? Making else just greed makes it consume the second expr_body. Making it any().optional() cause infinite loop hang. Any solutions to this?

Answer 1

Probably you want to use the a.delimitedBy(b) operator, which gives you a parser that consumes a one or more times separated and possibly ended by the argument b. If you need more control, have a look at how it is implemented.