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Home » Modeling » TMF (Xtext) » XText grammar with lookbehind
XText grammar with lookbehind [message #1756248] Tue, 14 March 2017 13:41 Go to next message
Stephan Pirnbaum is currently offline Stephan PirnbaumFriend
Messages: 1
Registered: March 2017
Junior Member

I am new in developing a DSL using XText and I am currently stuck in a problem.

I have to parse a file which contains elements called Class and might look like this:
A {
B, //this comma is needed
D {E}, //comma is optional after a closing curly brace
F {G}

As you can see, a comma is a must after a class without subclasses (those enclosed by curly brackets) when another class follows. Otherwise, the comma is optional.

My current (wrong) solution is:
Model: (classes += Class)*
Class: name = ID ('{' (subclasses += Class) (',' subclasses += Class)* '}')?

Sadly, this works only when the comma is a must in any case. I believe the grammar should express the following structure:
Class: ID (',' Class | '{' subclasses += Class '}' ','? Class)?

Of course, that does not work since I need to assign the return value of Class to the subclasses of the parent. After some looking around different blogs, I belive that using a lookbehind would be a solution. However, I havn't found a way to express this in XText.

Is there a solution or a workaround for this issue?

Thanks in advance
Re: XText grammar with lookbehind [message #1756272 is a reply to message #1756248] Tue, 14 March 2017 18:51 Go to previous message
Christian Dietrich is currently offline Christian DietrichFriend
Messages: 11960
Registered: July 2009
Senior Member
Did you try something like

	((classes+=ClassWithBraces|classes+=ClassWithComma)* classes+=ClassAtEnd?);

ClassWithComma returns Class:
	name=ID ","

ClassWithBraces returns Class:
	name=ID =>("{" ((classes+=ClassWithBraces|classes+=ClassWithComma)* classes+=ClassAtEnd?) "}") ","?

ClassAtEnd returns Class:

(you may need to add a validation if you dont want a "," at the very end

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[Updated on: Tue, 14 March 2017 19:11]

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