void checkInvalidRuleRef(Token ruleID) {
		String fileName = ruleID.getInputStream().getSourceName();
		if ( g.isLexer() && Character.isLowerCase(ruleID.getText().charAt(0)) ) {
			g.tool.errMgr.grammarError(ErrorType.PARSER_RULE_REF_IN_LEXER_RULE,
									   fileName, ruleID, ruleID.getText(), currentRuleName);
		}
	}
Value of caseInsensitive option (false if not defined)
tool/src/org/antlr/v4/semantics/BasicSemanticChecks.java