$1 don't reset (non loop)

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

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Re: $1 don't reset (non loop) by snowcrash (Friar) on Feb 17, 2004 at 11:20 UTC
from the perlre manpage: The numbered variables ($1, $2, $3, etc.) and the related punctuation set ("$+", "$&", "$`", and "$'") are all dynamically scoped until the end of the enclosing block or until the next successful match, whichever comes first. So $1 is only set if the match succeeds, that's why it's a good idea to write `if ($adress1 =~ /^([A-Za-z0-9鲣逯呐][A-Za-z鲣逯呐0-9\s\-\.\,])$/) { $adress1 = $1; } if ($adress2 =~ /^([A-Za-z0-9鲣逯呐][A-Za-z鲣逯呐0-9\s\-\.\,])$/) { $adress2 = $1; }` [download] snowcrash	[reply] [d/l]
Re: Re: $1 don't reset (non loop) by Skeeve (Parson) on Feb 17, 2004 at 11:35 UTC
If I see it right, you didn't get it quite correct. $addressX should become undef if no match. So this should work: `if ($adress1 =~ /^([A-Za-z0-9鲣逯呐][A-Za-z鲣逯呐0-9\s\-\.\,])$/) { $adress1 = $1; } else { $adress1 = undef; } # and so on...` [download] ~~which should be reducable to:~~ Which is not, as you'll see below, reducable to: `{ $adress1 =~ /^([A-Za-z0-9鲣逯呐][A-Za-z鲣逯呐0-9\s\-\.\,])$/) $adress1 = $1; } { $adress2 =~ /^([A-Za-z0-9鲣逯呐][A-Za-z鲣逯呐0-9\s\-\.\,])$/) $adress2 = $1; }` [download] Update:* Thanks to Abigail-II who found out my mistake.	[reply] [d/l] [select]
Re: $1 don't reset (non loop) by Abigail-II (Bishop) on Feb 17, 2004 at 11:56 UTC
No, not really. That only works if `$1` happens to be undefined. Which you can't count on. Watch: `#!/usr/bin/perl use strict; use warnings; "meep" =~ /(.)/; # Set $1. my $address1 = "Tsjakka"; # Will match. my $address2 = "Tsjakka!"; # Will not match. { $address1 =~ /^([A-Za-z0-9鲣逯呐][A-Za-z鲣逯呐0-9\s\-\.\,])$/; $address1 = $1; } { $address2 =~ /^([A-Za-z0-9鲣逯呐][A-Za-z鲣逯呐0-9\s\-\.\,])$/; $address2 = $1; } print $address1, "\n"; print $address2, "\n"; __END__ Tsjakka meep` [download] $address2 doesn't become undef. The solution is much simpler. The regex will either match the entire string, or there's no match. So, we could just do: `/^([A-Za-z0-9鲣逯呐][A-Za-z鲣逯呐0-9\s\-\.\,])$/ or $_ = undef for $address1, $address2;` [download] Abigail	[reply] [d/l] [select]
Re: $1 don't reset (non loop) by Abigail-II (Bishop) on Feb 17, 2004 at 11:42 UTC
Why doesn't $1 reset when the regexp doesn't match anything? Just because it was designed that way. `$1` will only be set on a succesful match, and hence remain what it is on a failed match. Abigail	[reply]
Re: $1 don't reset (non loop) by Skeeve (Parson) on Feb 17, 2004 at 11:31 UTC
How about: `($address1)= $adress1 =~ /^([A-Za-z0-9鲣逯呐][A-Za-z鲣逯呐0-9\s\-\.\,] +)$/; ($address2)= $adress2 =~ /^([A-Za-z0-9鲣逯呐][A-Za-z鲣逯呐0-9\s\-\.\,] +)$/;` [download]	[reply] [d/l]
Re: Re: $1 don't reset (non loop) by jockel (Beadle) on Feb 17, 2004 at 13:03 UTC
Hi all This would be the cleanest looking sollution.. but will it work.. I'll try this.. Thanks everyone! /jocke	[reply]
Re: Re: Re: $1 don't reset (non loop) by Skeeve (Parson) on Feb 17, 2004 at 22:04 UTC
Another "clean looking solution"!? `($_)= /^([A-Za-z0-9鲣逯呐][A-Za-z鲣逯呐0-9\s\-\.\,]*)$/ for ($address1 +, $address2);` [download] Works by aliasing the current variable to $_	[reply] [d/l]
Re: $1 don't reset (non loop) by MCS (Monk) on Feb 17, 2004 at 13:23 UTC
Why doesn't $1 reset when the regexp doesn't match anything? It's a feature, not a bug :-) Actually there are times when it comes in handy, if I'm doing stuff where I need it to be redefined I usually use an if {} elsif{} else{} loop. That way it only goes into the {} if the if (or elsif) matches. If you don't want to use an elsif (because you are checking a few things) just use multiple if statements.	[reply]
Re: Re: $1 don't reset (non loop) by jockel (Beadle) on Feb 17, 2004 at 14:01 UTC
Well... this time I think it's a bug =).. It would've been great if you could define this so called "feature" with a special $ variable. Anyway .. now when I know about this "feature" and won't make the mistake again =) /jocke	[reply]