[perlre] $_ = /$re/ vs $

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

What is the functional difference between the $_ = line and the $_ =~ line line of code? Besides one being commented out... hahahah very funny :) The latter is a patch that I got that he was happy with and it passes my test case, so I never bothered about it, but now I have some free time and was wondering what the diff was in the 2 lines.

foreach (@{$dir})
        {
        # $_ =~ m#([a-z-]*)\s*([0-9]*)\s*([0-9a-zA-Z]*)\s*([0-9a-zA-Z]
+*)\s*([0-9]*)\s*([A-Za-z]*)\s*([0-9]*)\s*([0-9A-Za-z:]*)\s*([A-Za-z0-
+9.-]*)#;
          $_ =  m#([a-z-]*)\s*([0-9]*)\s*([0-9a-zA-Z]*)\s*([0-9a-zA-Z]
+*)\s*([0-9]*)\s*([A-Za-z]*)\s*([0-9]*)\s*([0-9A-Za-z:]*)\s*([\w*\W*\s
+*\S*]*)#;

        my $perm = $1;
        my $inode = $2;
        my $owner = $3;
        my $group = $4;
        my $size = $5;
        my $month = $6;
        my $day = $7;
        my $yearOrTime = $8;
        my $name = $9;
        my $linkTarget;
[download]

Carter's compass: I know I'm on the right track when by deleting something, I'm adding functionality... download and use The Emacs Code Browser

Comment on [perlre] $_ = /$re/ vs $_ =~ /$re/ Select or Download Code

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Re: [perlre] $_ = /$re/ vs $_ =~ /$re/ by MarkM (Curate) on Aug 28, 2003 at 23:49 UTC
Note that "$_ = /$re/" is actually equivalent to "$_ = ($_ =~ /$re/)". The final result that I would expect, is for ($_ =~ /$re/) to be interpretted in scalar context, resulting in a boolean value (1 or ""), and for the boolean value to be assigned to $_, which will alter the elements in @$dir to each be either 1 or "", depending on whether the element matched the regular expression or not. Since you don't seem to be using @$dir again, this may have gone unnoticed. Since both expression invoke '$_ =~ /$re/', they do indeed 'do the same thing'. If you are trying to reduce characters, try just "/$re/" without any mention of $_. As others recommended, you should really use "if (/$re/) {", as the match may not succeed.	[reply]
Re: [perlre] $_ = /$re/ vs $_ =~ /$re/ by revdiablo (Prior) on Aug 29, 2003 at 01:50 UTC
MarkM has directly answered your question, but I thought it might be handy to point out that ~~`m///`~~ `m//` in list context returns all the subexpressions matched in parens. I think this would be useful in your example code. e.g.: `my ($perm, $inode, $owner, $group, $size, $month, $day, $yearOrTime, $name) = m!$re_with_capturing_parens! or next;` [download] Note: the `or next` at the end of this satisfies the recommendation to skip any lines that don't match. Update: make that `m//` instead of `m///` :-)	[reply] [d/l] [select]
Re: [perlre] $_ = /$re/ vs $_ =~ /$re/ by runrig (Abbot) on Aug 28, 2003 at 23:41 UTC
The first just calls the match operator on $_, and the second version does the same and then sets $_ to the empty string or 1. Either way, there really should be a test to see if the match is successful before you go using the $1, $2, etc. variables.	[reply]