(flex.info)Why can't flex understand this variable trailing context pattern?

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Why can't flex understand this variable trailing context pattern?
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     To: Gregory Margo <gmargo@newton.vip.best.com>
     Subject: Re: flex-2.5.3 bug report
     In-reply-to: Your message of Sat, 23 Nov 1996 16:50:09 PST.
     Date: Sat, 23 Nov 1996 17:07:32 PST
     From: Vern Paxson <vern>
     
     > Enclosed is a lex file that "real" lex will process, but I cannot get
     > flex to process it.  Could you try it and maybe point me in the right direction?
     
     Your problem is that some of the definitions in the scanner use the '/'
     trailing context operator, and have it enclosed in ()'s.  Flex does not
     allow this operator to be enclosed in ()'s because doing so allows undefined
     regular expressions such as "(a/b)+".  So the solution is to remove the
     parentheses.  Note that you must also be building the scanner with the -l
     option for AT&T lex compatibility.  Without this option, flex automatically
     encloses the definitions in parentheses.
     
     		Vern