= rather than =~ ?

bcrowell2 has asked for the wisdom of the Perl Monks concerning the following question:

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•Re: = rather than =~ ? by merlyn (Sage) on Jan 01, 2004 at 17:21 UTC
Good catch! That should have been `=~`! Where were you when that was being tech reviewed? Or where was my brain when I was typing that? Thanks... I'll add it as an errata. -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply]
Re: •Re: = rather than =~ ? by pg (Canon) on Jan 01, 2004 at 20:15 UTC
This note might confuse newbies, so I thought just add a comment. "That should have been =~!" What merly said is that, that was a typo, and not what he wanted. (As you can see, he is the owner of that web page.) But as a Perl statement, it is absolutely valid, and jweed has already given a full explanation.	[reply]
Re: •Re: = rather than =~ ? by diotalevi (Canon) on Jan 01, 2004 at 23:01 UTC
Goodness. And all this time I thought you had /meant/ it that way.	[reply]
Re: = rather than =~ ? by jweed (Chaplain) on Jan 01, 2004 at 17:18 UTC
This is technically valid perl. Since no search string is specified, $_ will be searched, implicitly. Then, as perlop points out, the match in scalar context returns true when a match occurs and false when it doesn't. So $input will be true or false, based on whether $_ had one or more word characters. Who is Kayser Söze? Code is (almost) always untested.	[reply]
Re: = rather than =~ ? by Anonymous Monk on Jan 01, 2004 at 17:17 UTC
It depends. I don't know what $input is, but that code is matching against $_. If the match fails, the match fails. Such code should always be written something like $foo = $1 if /(foo)/;	[reply]
Re: Re: = rather than =~ ? by tachyon (Chancellor) on Jan 02, 2004 at 03:17 UTC
`my $foo = m/(foo)/ ? $1 : '';` [download] cheers tachyon	[reply] [d/l]
Re: Re: = rather than =~ ? by ysth (Canon) on Jan 02, 2004 at 04:11 UTC
`$foo = $1 if /(foo)/;` [download] may be okay, but `my $foo = 1 if /(foo)/;` [download] is not.	[reply] [d/l] [select]
Re^3: = rather than =~ ? (too brief?) by Aristotle (Chancellor) on Jan 02, 2004 at 04:22 UTC
If you feel the need point that one out, you should probably also explain why that is.. :) Makeshifts last the longest.	[reply]
Re^4: = rather than =~ ? (too brief?) by Coruscate (Sexton) on Jan 02, 2004 at 05:43 UTC
Actually, it's worse than that by mrpeabody (Friar) on Jan 02, 2004 at 19:44 UTC
Re^5: = rather than =~ ? (too brief?) by Aristotle (Chancellor) on Jan 02, 2004 at 22:00 UTC
Re: Re^4: = rather than =~ ? (too brief?) by BrowserUk (Patriarch) on Jan 02, 2004 at 20:31 UTC
Re: = rather than =~ ? by SavannahLion (Pilgrim) on Jan 02, 2004 at 07:09 UTC
I just wanted to say thank you for pointing something like that that is so fundamentally important. After reading your example and better understanding how $1 behaved, I went back and took a careful look over some source code. There were three distinct sections where I made that error. After I examined the code logic, there was indeed one circumstance where, if the match failed, then there was a potential for $1 to be exposed. (The other two are wrapped in if statements.) Fortunately, I took to reading PerlMonks on a regular basis and happened to read this post before the code went, "live." There's no way I can give all of my daily allocated points to just the people in this thread, but if I could, I would. ---- Thanks for your patience. Prove your knowledge @ HLPD Edit: Fixed botched HTML :) Sorry everyone has a different markup language.	[reply]
Re: = rather than =~ ? by inman (Curate) on Jan 02, 2004 at 18:04 UTC
Another potentially confusing variation would be if list context was used since the match will return the list of matched sub-expressions. In the example below, $input is assigned a value for each iteration wheras $1 is only changed on a good match. `#! /usr/bin/perl use strict; my $input; while (<DATA>) { ($input) = /(\w+)/; print "\$input=$input\t\$1=$1\n"; } __DATA__ Hello World` [download] Output: `$input=Hello $1=Hello $input= $1=Hello $input=World $1=World` [download]	[reply] [d/l] [select]