XeTeX: How to *globally* replace an ASCII character with another symbol?

Question

A few months ago, I asked whether the “EB Garamond” font contains a short-arched version of the letter f. I have now found out how to access this letter because it actually exists. Writing

\newcommand\fshort{\XeTeXglyph\the\XeTeXglyphindex "f.short" \relax}

makes it possible to access this letter by writing \fshort.

Writing

\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\myreplace}{m}
{
    \tl_set:Nn \l__maxd_argument_tl { #1 }
    \tl_replace_all:Nnn \l__maxd_argument_tl { f } { \fshort }
    \tl_use:N \l__maxd_argument_tl
}
\tl_new:N \l__maxd_argument_tl
\ExplSyntaxOff

will result in the replacement of f with the short-arched version of it when the text is encompassed by \myreplace{}.

This, however, is not very convenient. I would like to globally replace every f character with the short-arched f.

I already tried using catcodes, but they do not work when it comes to (certain) ASCII characters. This answer which contains Cyrillic characters works perfectly fine:

\catcode`\Ё=\active
\defЁ{E}
\catcode`\ё=\active
\defё{e}

But writing something like

\catcode`\f=\active
\deff{\fshort}

or

\catcode`\f=\active
\def f{\fshort}

does not work. Also this question did not solve my problem.

---

David Carlisle’s solution works!

If you also want to use Q.short in combination with f.short, use the following code which has to be written in that order:

\newcommand\fshortgobble[1]{\XeTeXglyph\the\XeTeXglyphindex "f.short" \relax}

\newXeTeXintercharclass\fclass
\XeTeXcharclass `f =\fclass
\XeTeXinterchartoks 0 \fclass  {\fshortgobble}
\XeTeXinterchartoks \fclass \fclass  {\fshortgobble}
\XeTeXinterchartoks 4095 \fclass  {\fshortgobble}

\newcommand\Qshortgobble[1]{\XeTeXglyph\the\XeTeXglyphindex "Q.short" \relax}

\newXeTeXintercharclass\Qclass
\XeTeXcharclass `Q =\Qclass
\XeTeXinterchartoks 0 \Qclass  {\Qshortgobble}
\XeTeXinterchartoks \Qclass \Qclass  {\Qshortgobble}
\XeTeXinterchartoks 4095 \Qclass  {\Qshortgobble}

Answer 1

You could use a character class

\documentclass{article}
\usepackage{fontspec}
\newcommand\fshortgobble[1]{\XeTeXglyph\the\XeTeXglyphindex "f.short" \relax}

\newXeTeXintercharclass\fclass
\XeTeXcharclass `f =\fclass
\XeTeXinterchartoks 0 \fclass  {\fshortgobble}
\XeTeXinterchartoks \fclass \fclass  {\fshortgobble}
\XeTeXinterchartoks 4095 \fclass  {\fshortgobble}
\begin{document}

\setmainfont{EB Garamond}

f   ff  difficult football diff biff fence

\XeTeXinterchartokenstate=1

f   ff  difficult football diff biff fence



\end{document}