document restriction on group names
diff --git a/Doc/libregex.tex b/Doc/libregex.tex
index f3df684..10abece 100644
--- a/Doc/libregex.tex
+++ b/Doc/libregex.tex
@@ -88,7 +88,8 @@
brackets, e.g. \code{'\e(<id>[a-z][a-z0-9]*\e)'}, the group can
be referenced by its name in arguments to the \code{group} method of
the resulting compiled regular expression object, like this:
-\code{p.group('id')}.
+\code{p.group('id')}. Group names may contain alphanumeric characters
+and \code{'_'} only.
\end{funcdesc}
\begin{excdesc}{error}