Xref: utzoo comp.lang.perl:5410 comp.sources.bugs:2925
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uupsi!kepler1!jwu
From: jwu@kepler.com (Jasper Wu)
Newsgroups: comp.lang.perl,comp.sources.bugs
Subject: Re: BUG in interpolating and/or substituting NULs in strings?
Message-ID: <589@kepler1.kepler.com>
Date: 21 May 91 16:51:50 GMT
References: <GNB.91May21123118@leo.bby.oz.au>
Reply-To: jwu@kepler.com (Jasper Wu)
Followup-To: comp.lang.perl
Organization: Kepler Financial Management, Ltd., Setauket, NY.
Lines: 70


Greg, from perl manpage on s///:

	...If the PATTERN evaluates to a null string, the most recent 
	successful regular expression is used instead....

In article <GNB.91May21123118@leo.bby.oz.au> gnb@bby.oz.au (Gregory N. Bond) writes:
>#! /usr/local/bin/perl
>
>$NUL = "\0";
>$SOH = "\1";
>$STX = "\2";
>$ETX = "\3";
>
>$_ = "data a${STX}data b${ETX}cc${NUL}${SOH}";
>
>	$p = $_;
>	$p =~ s/$ETX/<ETX>/og;
>	$p =~ s/$STX/<STX>/og;
>	$p =~ s/$SOH/<SOH>/og;
>	$p =~ s/$NUL/<NUL>/og;
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>	print "[$p]\n";
>
>	$p = $_;
>	$p =~ s/$NUL/<NUL>/og;
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>	$p =~ s/$SOH/<SOH>/og;
>	$p =~ s/$STX/<STX>/og;
>	$p =~ s/$ETX/<ETX>/og;
>	print "[$p]\n";
>

both lines have $NUL evalualtes to null string, and happened to be equivalent 
to
	$p =~ s/$SOH/<SOH>/og;
that's why you got the (undesirable) result.

>leo% perl t.perl
>[data a<STX>data b<ETX>cc^@<SOH>]
>[data a<STX>data b<ETX>cc^@<NUL>]
>


To get what you want, simply not to evaluate the pattern to null string.  
You can either
	1) change those two lines to 	$p =~ s/\0/<NUL>/og;
or (preferrably)
	2) change the first four lines to
		$NUL = \0;
		$SOH = \1;
		$STX = \2;
		$ETX = \3;
	(ie, no double quote) and keep rest of the program intact.

Both ways worked fine when i tested it on my machine.
Hope this helps.


>Greg.
>--
>Gregory Bond, Burdett Buckeridge & Young Ltd, Melbourne, Australia
>Internet: gnb@melba.bby.oz.au    non-MX: gnb%melba.bby.oz@uunet.uu.net
>Uucp: {uunet,pyramid,ubc-cs,ukc,mcvax,prlb2,nttlab...}!munnari!melba.bby.oz!gnb


--jasper
============================
Jasper Wu   jwu@kepler.com