# stringstrings replacement for XeTeX?

To construct a lower-case filename out of the mixed-case argument given to a command, I used stringstrings in LaTeX.

Now I switched to XeTeX and it seems the fontspec-package has some interference with stringstrings:

\documentclass{article}
\usepackage{fontspec,stringstrings}
\begin{document}
\caselower[q]{AbC}\thestring
\end{document}

Produces an error message only when fontspec is used:

! Missing control sequence inserted.
<inserted text>
\inaccessible
l.4 \thestring

Just using \lowercase for display won't work, because I need to use it like

\edef{\filename}{stuff/\thestring.tex}
\input{\filename}

(there seems to be a similar clash between stringstrings and polyglossia)

So: what would be other string manipulation packages for LaTeX and XeTeX (coolstr is mentioned, but only supports substring operations I don't need)? Or is there a fix by redefining something?

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Your example works on my machine with current TeX Live. –  Philipp Dec 23 '10 at 0:05
Then my Ubuntu version of TeX Live is outdated… (looks like they have a reputation for lagging horribly behind with TeX versions) Thanks. –  pascal Dec 31 '10 at 13:04