Check for missing fonts/characters in XeLaTeX?

Question

Because I began to use many Unicode characters (Japanese, some unusual diacritics, etc.) I switched from LaTeX to XeLaTeX to render my documents. Unfortunately, if a font is missing, the letters are simply left out (for example, when I had forgotten to specify the font for Japanese) without any error (and I can't seem to spot a warning either). I will read the result, but I prefer to get some early warning if some character isn't available. Is there any way to get a warning about characters that have not been rendered? An error would be even better.

Answer 1

The log should list all missing characters in the form of:

Missing character: There is no <char> in font <font>

Where <char> and <font> is real character and TeX font name respectively. I usually use a simple grep call to check for such messages in the log.

Answer 2

It is easy. Just try:

\documentclass{article}
\usepackage{fontspec}
\newfontfamily\testfont[ExternalLocation]{persian-modern-regular}
\begin{document}
\setbox0=\hbox{\testfont A}
\ifdim\ht0=0pt Character A does not exist in font \else Character A exits in the font\fi
\end{document}

In the example above I used Persian Modern font which does not have Latin Characters so I get a box of height 0pt for character A.

Another one:

\documentclass{article}
\usepackage{fontspec}
% font spec loads "LAtin Modern" font which does not contain Persian characters
\begin{document}
\setbox0=\hbox{و}
\ifdim\ht0=0pt Character Vav does not exist in font \else Character Vav exits in the font\fi
\end{document}

Edit1: Based on Khaled comments "If there is a missing glyph in the font XeTeX will show the .notdef glyph, most fonts have a .notdef glyph with non zero height and width". So we can perform a furthur test with characters from a script that is unlikely to be found in every font. Say, Avestan (an ancient Iranian script). Being a Zoroastrian myself, there are only few unicode fonts that contain Avestan chracters. So let's try (although there may be a better approach):

\documentclass{article}
\usepackage{fontspec}
\newfontfamily\testfont[ExternalLocation]{amiri-regular}
\begin{document}
\setbox0=\hbox{\testfont A}
\setbox1=\hbox{\testfont \char"10B00} % this is Avestan character A
\ifvoid0 
Font does not have character A
\else
\ifdim\dp0=\dp1
\ifdim\ht0=\ht1
\ifdim\wd0=\wd1
Font does not have character A
\fi\fi\else
Font has character A
\fi\fi
\end{document}

Edit2: It seems that my previous answers was not needed because etex provides \iffontchar primitive which has the following syntax:

\iffontchar ⟨font⟩ ⟨code⟩ ⟨true text⟩ \else ⟨false text⟩ \fi

The primitive \iffontchar can be used to check whether a certain glyph exists in a font. For this purpose it takes a font and the code of a character and performs the test. If the character exists the then branch is expanded otherwise the else branch.

An example would be:

\documentclass{article}
\usepackage{fontspec} % this loads Latin Modern font
\begin{document}
Font \iffontchar\font`ج  has Persian character Jim\else  does not have Persian character Jim\fi
\end{document}