Well, I can make a ruleset that seems to fit:
  1. Always keep the first two numbers intact
  2. If any zeros start at position 3, remove them all.
  3. If you didn't do that, then if two or more zeros start after position 3, truncate them to a single 0
  4. If you didn't do either of the first two techniques, then remove a single 0 starting at position 3 or later entirely.

What happens if a single 0 starts at position 3? What does '35010333356' turn into?

Here's my code: 
#!/usr/bin/perl

use strict;
use warnings;
use Test::More;

my %output_for
    = ( '215000007801' => '21507801',
        '300000324002' => '30324002',
        '890000457651' => '89457651',
        '210004563401' => '214563401',
        '201045139158' => '20145139158', );

plan tests => scalar keys %output_for;

while ( my ($input, $correct_output) = each %output_for ) {
    my $orig_input = $input;
    $input = &compress_numstring( $input );
    is( $input, $correct_output, "Solved '$orig_input'" );
}

sub compress_numstring {
  my $starting_num = shift;
  return $starting_num
   unless(defined $starting_num && $starting_num =~ /^(\d{2})(\d+)/);
  my( $keep, $modify ) = ($1,$2);
  ( $modify =~ s/^0+// ) ||
  ( $modify =~ s/0{2,}/0/ ) ||
  ( $modify =~ s/(?<!0)0([1-9]|$)/$1/ );
  return $keep . $modify;
}

[download]
It passes the validation test, but it may fail further tests since the rules aren't clearly spelled out.

In reply to Re: Stuck in komplexer regex, at least for me by saintly
in thread Stuck in komplexer regex, at least for me by ultibuzz

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