Case insensitive lexer

Signed-off-by: Lantao Jin <ltjin@amazon.com>

Author: LantaoJin

ppl/src/main/antlr/OpenSearchPPLLexer.g4

@@ -7,7 +7,7 @@
 lexer grammar OpenSearchPPLLexer;
 
 channels { WHITESPACE, ERRORCHANNEL }
-
+options { caseInsensitive = true; }
 
 // COMMAND KEYWORDS
 SEARCH:                             'SEARCH';
@@ -416,7 +416,7 @@ INTEGER_LITERAL:                    DEC_DIGIT+;
 DECIMAL_LITERAL:                    (DEC_DIGIT+)? '.' DEC_DIGIT+;
 
 fragment DATE_SUFFIX:               ([\-.][*0-9]+)+;
-fragment CLUSTER_PREFIX_LITERAL:    [*a-zA-Z]+?[*a-zA-Z_\-0-9]* COLON;
+fragment CLUSTER_PREFIX_LITERAL:    [*A-Z]+?[*A-Z_\-0-9]* COLON;
 ID_DATE_SUFFIX:                     CLUSTER_PREFIX_LITERAL? ID_LITERAL DATE_SUFFIX;
 DQUOTA_STRING:                      '"' ( '\\'. | '""' | ~('"'| '\\') )* '"';
 SQUOTA_STRING:                      '\'' ('\\'. | '\'\'' | ~('\'' | '\\'))* '\'';
@@ -425,7 +425,7 @@ fragment DEC_DIGIT:                 [0-9];
 
 // Identifiers cannot start with a single '_' since this an OpenSearch reserved
 // metadata field.  Two underscores (or more) is acceptable, such as '__field'.
-fragment ID_LITERAL:                ([@*a-zA-Z_])+?[*a-zA-Z_\-0-9]*;
+fragment ID_LITERAL:                ([@*A-Z_])+?[*A-Z_\-0-9]*;
 
 
 ERROR_RECOGNITION:                  .    -> channel(ERRORCHANNEL);