While working with Antlr3 grammar, I have come across a situation when the rule's intent is effectively same but behaves differently.
I have created a small example-
I want to parse a qualified object name which may be 3-part or 2-part or unqualified (Dot is the separator).
Test Input-
1. SCH.LIB.TAB1;
2. LIB.TAB1;
3. TAB1;
I changed the below rule from having optionals to having alternatives (ORed rules).
Before State-
qualified_object_name
:
( identifier ( ( DOT identifier )? DOT identifier )? )
;
After State-
qualified_object_name_new
:
( identifier DOT identifier DOT identifier ) // 3 part name
| ( identifier DOT identifier ) // 2 part name
| ( identifier ) // 1 part name
;
Input 1 is parsed correctly by both the rules, but the new rule gives error while parsing input 2 and 3.
line 1:22 no viable alternative at input ';'
I assumed that Antlr will try to match against alternative 1 of qualified_object_name_new, but when does not match alternative 1 fully, then would try to match alternative 2 and so on.
So, for input 'LIB.TAB1' it would finally match against alternative 2 of qualified_object_name_new.
However, it is not working this way and gives error while paring 2-part name or unqualified name.
Interestingly, when I set option k = 1, then all 3 inputs are parsed correctly by the new rule. But with any other value of k, it gives error.
I want to understand why Antlr behaves this way and is this correct.
