Consider the below unicode table. Some slots are undefined by the Unicode Consortium and appear empty in the table (slot 7,8,15 &16 in second row). How can I detect these and replace the characters with a symbol?

Minimal:
\documentclass{article}
\usepackage{fontspec}
\newfontfamily\greek{Times New Roman}
\makeatletter
%puts the unicode label (removes last char and adds x)
\def\putunicode@label#1#2;{%
\def\reformat@unicode@string##1{%
\bgroup%
\ttfamily
U+%
\edef\z{}%
\expandafter\@tfor\expandafter\i\expandafter:\expandafter=#2;\do{%
\if\i;x\space
\else\z\fi%
\edef\z{\i}%
}%
\egroup
}%
\makebox[5em]{\reformat@unicode@string{#2}}%
}
\def\putchar@cx#1{%
\char\n$_{\the\r}$
}
\def\urow@cx#1{%
\parindent0pt%
\n=#1%
\r=1%
\expandafter\putunicode@label#1;%
\loop%
\ifnum\n<\numexpr#1+16%
\makebox[2.1em]{\expandafter\putchar@cx{#1}}%
\advance\r by1%
\ifnum\r>16\r=1\relax\else\fi
\advance\n by1%
\repeat
\par
}
\def\typeseturows@cx#1{%
\@for\next:=#1\do{%
\urow@cx\next\vskip3pt}}
\newcommand\unicodetable[2]{%
\par\leavevmode
\bgroup%
\parindent0pt%
\newcount\n%
\newcount\r%
\r=1%
\raggedright%
\@nameuse{#1}%
\r=1%
{\ttfamily\makebox[3.1em]{}}%
\loop%
\ifnum\r<17%
\makebox[2.1em]{\hfill\texttt{\the\r}\hfill}%
\advance\r by1%
\repeat
\vskip3pt
\typeseturows@cx{#2}%
\egroup%
}
\makeatother
\begin{document}
\unicodetable{greek}{"1F00,"1F10,"1F20,"1F60}
\end{document}

%, which are innocuous, though. – egreg Sep 21 '14 at 19:27