regex question

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

Hi,

I would like to turn a multiline string that looks like

1 A
2 B
[download]

into a hash that looks like

{
  1 => "A",
  2 => "B", 
}
[download]

And I tought this was easy enough, however to my surprise this does not work:

use strict; 
use Data::Dumper;

my $s = "1 A\n2 B\n"; 

my %h =map { $1 => $2 } $s =~ /(\d+)\s+(\S+)/mg;
print Dumper(\%h);
[download]

It produces

$VAR1 = {
          '2' => 'B'
        };
[download]

Could someone please explain to me what is going on here?

Many thanks!

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Re: regex question by Corion (Patriarch) on Apr 01, 2020 at 17:16 UTC
The match runs to completion before the `map` even starts. At the time your `map` runs, `$1` and `$2` are what they are at the end of the global match. Most likely you want `pairs` from List::Util: `foreach my $pair ( pairs /(\d+)\s+(\S+)/mg ) { my ( $key, $value ) = @$pair; ... }` [download] ... or a while loop: `$h{ $1 } = $2 while /(\d+)\s+(\S+)/mg;` [download]	[reply] [d/l] [select]
Re: regex question by AnomalousMonk (Archbishop) on Apr 01, 2020 at 17:51 UTC
Or even more simply: `c:\@Work\Perl\monks>perl -wMstrict -le "use Data::Dumper; ;; my $s = qq{1 A\n2 B\n}; ;; my %h = $s =~ /(\d+)\s+(\S+)/g; print Dumper(\%h); " $VAR1 = { '1' => 'A', '2' => 'B' };` [download] Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: regex question by AnomalousMonk (Archbishop) on Apr 02, 2020 at 01:45 UTC
morgon: Further to Corion's post: `[Corion]:` The match runs to completion before the `map` even starts. At the time your `map` runs, `$1` and `$2` are what they are at the end of the global match. Here's an instrumented version of the OPed code to better (I hope) show what's happening: `c:\@Work\Perl\monks>perl -wMstrict -le "use Data::Dumper; ;; my $s = \"1 A\n2 B\n33 foo\n\"; ;; my %h = map { print qq{in map: \$_ '$_' \$1 '$1' \$2 '$2'}; $1 => $2; } $s =~ /(\d+)\s+(\S+)/mg ; print Dumper(\%h); " in map: $_ '1' $1 '33' $2 'foo' in map: $_ 'A' $1 '33' $2 'foo' in map: $_ '2' $1 '33' $2 'foo' in map: $_ 'B' $1 '33' $2 'foo' in map: $_ '33' $1 '33' $2 'foo' in map: $_ 'foo' $1 '33' $2 'foo' $VAR1 = { '33' => 'foo' };` [download] (Note that this example doesn't take into account the revised data format specification suggested here. :) Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: regex question by hippo (Bishop) on Apr 01, 2020 at 17:53 UTC
As Corion has answered your question, here's just a simpler (IMHO) approach to the task: `#!/usr/bin/env perl use strict; use warnings; use Data::Dumper; my $s = "1 A\n2 B\n"; my %h = split /\s/, $s; print Dumper(\%h);` [download]	[reply] [d/l]
Re^2: regex question by morgon (Priest) on Apr 01, 2020 at 19:34 UTC
Very imaginative, but in reality my lines look not like "1 A" but rather like "1 string1 string2 string3" and so your approach would not work anymore. But your idea is cool.	[reply]
Re^3: regex question by AnomalousMonk (Archbishop) on Apr 01, 2020 at 23:08 UTC
Clear questions elicit clear and helpful answers. What should the content of your hash look like given the new data format? Do any of the other solutions given so far produce that ~~structure~~ \| hash content? Give a man a fish: `<%-{-{-{-<`	[reply] [d/l]
Re^3: regex question by rsFalse (Chaplain) on Apr 02, 2020 at 15:17 UTC
If you want to match "1 string1" from "...\n1 string1 string2 string3\n...", use anchor '^', otherwise modifier 'm' is redundant. Did it help?	[reply]
Re: regex question by tybalt89 (Monsignor) on Apr 01, 2020 at 17:51 UTC
`#!/usr/bin/perl use strict; # https://perlmonks.org/?node_id=11114912 use warnings; my $s = "1 A\n2 B\n"; my %h = split ' ', $s; use Data::Dump 'dd'; dd \%h;` [download]	[reply] [d/l]


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