Using substr as an alternative to regex substitution and also grouping in fours from the right-hand side in case that is a requirement.

$ perl -Mstrict -Mwarnings -E '
sub ins
{
    my $str = shift;
    my $len = length $str;
    while ( ( $len -= 4 ) > 0 )
    {
        substr $str, $len, 0, q{ };
    }
    return $str;
}

say ins( $_ ) for qw{
   1
   10
   110
   1101
   10010
   101100
   1110101
   10001011
   110100010
   };'
1
10
110
1101
1 0010
10 1100
111 0101
1000 1011
1 1010 0010
$

[download]

I hope this is useful.

Cheers,

JohnGG

In reply to Re: Add space in long string by johngg
in thread Add space in long string by lddzjwwy

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