How to match a string, but case-insensitively?

Question

Let\'s say that I want to match \"beer\", but don\'t care about case sensitivity.

Currently I am defining a token to be (\'b\'|\'B\' \'e\'|\'E\' \'e\'|\'E\' \'r\'|

Answer 1

A solution I used in C#: use ASCII code to shift character to smaller case.

class CaseInsensitiveStream : Antlr4.Runtime.AntlrInputStream {
  public CaseInsensitiveStream(string sExpr)
     : base(sExpr) {
  }
  public override int La(int index) {
     if(index == 0) return 0;
     if(index < 0) index++;
     int pdx = p + index - 1;
     if(pdx < 0 || pdx >= n) return TokenConstants.Eof;
     var x1 = data[pdx];
     return (x1 >= 65 && x1 <= 90) ? (97 + x1 - 65) : x1;
  }
}

Answer 2

I would like to add to the accepted answer: a ready -made set can be found at case insensitive antlr building blocks, and the relevant portion included below for convenience

fragment A:('a'|'A');
fragment B:('b'|'B');
fragment C:('c'|'C');
fragment D:('d'|'D');
fragment E:('e'|'E');
fragment F:('f'|'F');
fragment G:('g'|'G');
fragment H:('h'|'H');
fragment I:('i'|'I');
fragment J:('j'|'J');
fragment K:('k'|'K');
fragment L:('l'|'L');
fragment M:('m'|'M');
fragment N:('n'|'N');
fragment O:('o'|'O');
fragment P:('p'|'P');
fragment Q:('q'|'Q');
fragment R:('r'|'R');
fragment S:('s'|'S');
fragment T:('t'|'T');
fragment U:('u'|'U');
fragment V:('v'|'V');
fragment W:('w'|'W');
fragment X:('x'|'X');
fragment Y:('y'|'Y');
fragment Z:('z'|'Z');

So an example is

   HELLOWORLD : H E L L O W O R L D;

Answer 3

How about define a lexer token for each permissible identifier character, then construct the parser token as a series of those?

beer: B E E R;

A : 'A'|'a';
B: 'B'|'b';

etc.

Answer 4

Define case-insensitive tokens with

BEER: [Bb] [Ee] [Ee] [Rr];

Answer 5

New documentation page has appeared in ANTLR GitHub repo: Case-Insensitive Lexing. You can use two approaches:

1. The one described in @javadba's answer
2. Or add a character stream to your code, which will transform an input stream to lower or upper case. Examples for the main languages you can find on the same doc page.

My opinion, it's better to use the first approach and have the grammar which describes all the rules. But if you use well-known grammar, for example from Grammars written for ANTLR v4, then second approach may be more appropriate.