in reply to Re-reading history from 2001 / using a capture during split

split is splitting on the un-captured 'B', and since it's not captured...

No, it's the (A) that's not captured. Every group produces a field, as last paragraph in split explains.

If the PATTERN contains capturing groups, then for each separator, an additional field is produced for each substring captured by a group (in the order in which the groups are specified, as per backreferences); if any group does not match, then it captures the undef value instead of a substring 
  DB<3> @foo = split /(A)|(B)/, "1A2B3"


  DB<4> x @foo
0  1
1  'A'
2  undef
3  2
4  undef
5  'B'
6  3

[download]

I'm reading linked node as japhy apologizes for what he did (i.e. making "split" to return empty string), and acknowledges that returning "undef" was correct from the very begining.

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Re^2: Re-reading history from 2001
by choroba (Cardinal) on Mar 05, 2019 at 17:14 UTC
    Yes, and to get rid of the undefs, one can use the branch reset pattern: 
    my @foo = split /(?|(A)|(B))/, "1A2B3";
use Data::Dumper; print Dumper \@foo;

    [download]
    $VAR1 = [
          '1',
          'A',
          '2',
          'B',
          '3'
        ];

    [download]
    map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
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