void checkTokenDefinition(Token tokenID) {
		String fileName = tokenID.getInputStream().getSourceName();
		if ( !Grammar.isTokenName(tokenID.getText()) ) {
			g.tool.errMgr.grammarError(ErrorType.TOKEN_NAMES_MUST_START_UPPER,
									   fileName,
									   tokenID,
									   tokenID.getText());
		}
	}
Value of caseInsensitive option (false if not defined)
tool/src/org/antlr/v4/semantics/BasicSemanticChecks.java