chen - merge into 1 grammar in parser

Signed-off-by: Jialiang Liang <jiallian@amazon.com>

Author: RyanL1997

ppl/src/main/antlr/OpenSearchPPLParser.g4

@@ -248,8 +248,7 @@ regexCommand
     ;
 
 regexExpr
-    : field=qualifiedName EQUAL pattern=stringLiteral
-    | field=qualifiedName NOT_EQUAL pattern=stringLiteral
+    : field=qualifiedName operator=(EQUAL | NOT_EQUAL) pattern=stringLiteral
     ;
 
 patternsMethod

ppl/src/main/java/org/opensearch/sql/ppl/parser/AstBuilder.java

@@ -910,7 +910,7 @@ public UnresolvedPlan visitAppendcolCommand(OpenSearchPPLParser.AppendcolCommand
   @Override
   public UnresolvedPlan visitRegexCommand(OpenSearchPPLParser.RegexCommandContext ctx) {
     UnresolvedExpression field = internalVisitExpression(ctx.regexExpr().field);
-    boolean negated = ctx.regexExpr().NOT_EQUAL() != null;
+    boolean negated = ctx.regexExpr().operator.getType() == OpenSearchPPLParser.NOT_EQUAL;
     Literal pattern = (Literal) internalVisitExpression(ctx.regexExpr().pattern);
 
     return new Regex(field, negated, pattern);