I have what I think is a simple ANTLR question. I have two token types: ident
and special_ident
. I want my special_ident
to match a single letter followed by a single digit. I want the generic ident
to match a single letter, optionally followed by any number of letters or digits. My (incorrect) grammar is below:
expr
: special_ident
| ident
;
special_ident : LETTER DIGIT;
ident : LETTER (LETTER | DIGIT)*;
LETTER : 'A'..'Z';
DIGIT : '0'..'9';
When I try to check this grammar, I get this warning:
Decision can match input such as "LETTER DIGIT" using multiple alternatives: 1, 2. As a result, alternative(s) 2 were disabled for that input
I understand that my grammar is ambiguous and that input such as A1
could match either ident
or special_ident
. I really just want the special_ident
to be used in the narrowest of cases.
Here's some sample input and what I'd like it to match:
A : ident
A1 : special_ident
A1A : ident
A12 : ident
AA1 : ident
How can I form my grammar such that I correctly identify my two types of identifiers?
Seems that you have 3 cases:
A
AN
A(A|N)(A|N)+
You could classify the middle one as special_ident
and the other two as ident
; seems that should do the trick.
I'm a bit rusty with ANTLR, I hope this hint is enough. I can try to write out the expressions for you but they could be wrong:
long_ident : LETTER (LETTER | DIGIT) (LETTER | DIGIT)+
special_ident : LETTER DIGIT;
ident : LETTER | long_ident;
Expanding on Carl's thought, I would guess you have four different cases:
Only option 2 should be token special_ident and the other three should be ident. All tokens can be identified by syntax alone. Here is a quick grammar I was able to test in ANTLRWorks and it appeared to work properly for me. I think Carl's might have one bug when trying to check AA , but getting you 99% there is a huge benefit, so this is only a minor modification to his quick thought.
prog
: (expr WS)+ EOF;
expr
: special_ident {System.out.println("Found special_ident:" + $special_ident.text + "\n");}
| ident {System.out.println("Found ident:" + $ident.text + "\n");}
;
special_ident : LETTER DIGIT;
ident : LETTER
|LETTER DIGIT (LETTER|DIGIT)+
|LETTER LETTER (LETTER|DIGIT)*;
LETTER : 'A'..'Z';
DIGIT : '0'..'9';
WS
: (' '|'\t'|'\n'|'\r')+;
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