bin/169500: /bin/expr improperly requires forward slash to be escaped

[Garrett Wollman]

<<On Thu, 28 Jun 2012 22:20:13 GMT, "Ronald F. Guilmette" <rfg at tristatelogic.com> said:

>  (These publications are quite expensive, as you may know, and thus, I have
>  not updated mine in quite some time.)
 
The current, complete POSIX specifications are available free by
registration at
<https://www2.opengroup.org/ogsys/jsp/publications/PublicationDetails.jsp?publicationid=12310>.

This is The Open Group's version of it; you can also pay lots of
money to get a printed document from ISO (ISO/IEC 9945:2009) or IEEE
(IEEE Std.1003.1-2008).

-GAWollman