# Undefined Macro in Package xstring

I have a very basic problem. I am trying to use the `xstring` package in a Latex document and I keep getting the following error message for an undefined control sequence:

``````(c:/Program Files (x86)/PCTeX/PCTeXv6/texmf-dist/tex/generic/xstring/xstring.sty

(c:/Program Files (x86)/PCTeX/PCTeXv6/texmf-dist/tex/generic/xstring/xstring.tex

****! Undefined control sequence.**

**\@xs@buildlines ...\expandafter {\number \numexpr
#1+1-1}\edef \@xs@reserved...**

**l.427 \@xs@newmacro\StrRemoveBraces{}{1}{1}
{%******

Typesetter process aborted
``````

I downloaded and installed this package from CTAN; it is the 2013-07-29 version.

Package `xstring` requires the e-TeX extensions. Otherwise you get the error, that `\numexpr` from e-TeX is undefined as seen in the question.
It seems that PCTeX has some support for e-TeX, but it must be enabled. This is done during format generation, if the first characters is a star `*`.
• +1 I just realised that I was thrown by the formatting (`\numexpr` wasn't at the end of the line, until we both edited the q at the same time:-) – David Carlisle Sep 23 '13 at 2:01