Add obsolescence note.
diff --git a/Doc/lib/libregsub.tex b/Doc/lib/libregsub.tex
index 6d48986..03e5c3f 100644
--- a/Doc/lib/libregsub.tex
+++ b/Doc/lib/libregsub.tex
@@ -7,6 +7,17 @@
Warning: these functions are not thread-safe.
+\strong{Obsolescence note:}
+This module is obsolete as of Python version 1.5; it is still being
+maintained because much existing code still uses it. All new code in
+need of regular expressions should use the new \code{re} module, which
+supports the more powerful and regular Perl-style regular expressions.
+Existing code should be converted. The standard library module
+\code{reconvert} helps in converting \code{regex} style regular
+expressions to \code{re} style regular expressions. (For more
+conversion help, see the URL
+\file{http://starship.skyport.net/crew/amk/regex/regex-to-re.html}.)
+
\renewcommand{\indexsubitem}{(in module regsub)}
\begin{funcdesc}{sub}{pat\, repl\, str}