item [1.]quad Identifierquad   program
            item [2.]quad White_Spacequad' '

documentclass{article}
usepackage{calc}
begin{document}
    begin{enumerate}
        item makebox[widthof{White_Space}][l]{Identifier}quad program
        item White_Space quad makebox[widthof{program}]{''}
    end{enumerate}
end{document}