UPDATE: see Re: Recursive sub-pattern spectacularly slow, what's wrong? (me, or just this use case, or a bug?) the following is no full solution

---

this

• $str =~ s/((\w)(?1)?\g{-1})//g;

does the trick without while loop, you need a relative \g{-1} reference when doing recursions.

for extra speed I'll do

• $str =~ s/((\w)(?1)?\g{-1}+)//g;

#  https://theweeklychallenge.org/blog/perl-weekly-challenge-340/
use v5.12;
use warnings;
use Test::More;

for my $case ('abbaca','azxxzy','aaaaaaaa','aabccba','abcddcba') {

  my $str = $case;
  say "*** Input: $str";
  while ( $str =~s/(\w)\1// ) {
    say "Remove '$&' => '$str'";
  } 
  my $exp = $str;
  say "Expected Output : '$str'";

  $str = $case;
  $str =~ s/((\w)(?1)?\g{-1})//g;
  say "Got Output : '$str'";
  is($exp,$str,"'$case' ->'$str'");

  say "...\n"
}

done_testing()
[download]

*** Input: abbaca
Remove 'bb' => 'aaca'
Remove 'aa' => 'ca'
Expected Output : 'ca'
Got Output : 'ca'
ok 1 - 'abbaca' ->'ca'
...

*** Input: azxxzy
Remove 'xx' => 'azzy'
Remove 'zz' => 'ay'
Expected Output : 'ay'
Got Output : 'ay'
ok 2 - 'azxxzy' ->'ay'
...

*** Input: aaaaaaaa
Remove 'aa' => 'aaaaaa'
Remove 'aa' => 'aaaa'
Remove 'aa' => 'aa'
Remove 'aa' => ''
Expected Output : ''
Got Output : ''
ok 3 - 'aaaaaaaa' ->''
...

*** Input: aabccba
Remove 'aa' => 'bccba'
Remove 'cc' => 'bba'
Remove 'bb' => 'a'
Expected Output : 'a'
Got Output : 'a'
ok 4 - 'aabccba' ->'a'
...

*** Input: abcddcba
Remove 'dd' => 'abccba'
Remove 'cc' => 'abba'
Remove 'bb' => 'aa'
Remove 'aa' => ''
Expected Output : ''
Got Output : ''
ok 5 - 'abcddcba' ->''
...

1..5
[download]

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