Hi, You need an anchor '^' to make sure the matchings start from the beginning of your strings..and your requirements might be written into two patterns which would be much easier to understand(the order of two s/// expressions matters).. 
#!/usr/bin/perl
use warnings;
use strict;

while(<DATA>) {
    s/^(\d\d[1-9])0(?=[1-9])/$1/;
    s/^(\d\d(?:[1-9]0)?)0+/$1/;

    print;
}

__DATA__
215000007801
300000324002
890000457651
210004563401
201045139158
Regards,
Xicheng

In reply to Re^2: Stuck in komplexer regex, at least for me by xicheng
in thread Stuck in komplexer regex, at least for me by ultibuzz

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