For the benefit of other monks who might be trying their hand at this: 

use Test::More;

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

plan tests => scalar keys %output_for;

sub solution {
    # (this doesn't work)
    $_[0] =~ s{ \A ([^0]+ 0) ([^0]*) 0+ ([^0]+0) }{$1$2$3}x
#        ||
#    $_[0] =~ s{ \A ([^0]+) 0+ ([^0]) }{$1$2}x;
}

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

[download]

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

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.