Hi, what you need is the longest prefix match from a list of variable length strings. One possible approach would be to create a hash of hashes of hashes... where at each level one digit of the given number is a key identifies an edge (of a labeled 10-ary tree?).
Another idea would be to look-up the prefixes from a simple hash starting with the longest substring from the phone number to check.

Something like this (too tired to clean it up, but it shall convey an idea): 

#!/bin/perl

use strict;
use warnings;

my %tab;
foreach (<DATA>) {
        chomp;
        $tab{$1}=$2  if /^\s*0(\d+)\s+(.*)/; # e.g. 30 -> Berlin
        # you can compute min/max of the prefix here (s. below)
}

                            # already normalised w/o leading 0
NUMBER: foreach my $num ( qw(204112345 3304123456 3145666 301234567) )
+ {  
            for (my $len=5; $len>=2; $len--) { 
                 # assumes max. prefix lenght is 5, min. is 2
                 my $prefix = substr($num,0,$len);
                 if (defined (my $town=$tab{$prefix})) {
                     printf "0%-12s = %5s-%-7s in sunny %s\n", 
                              $num,  "0$prefix", 
                              substr($num,$len), $town;
                     next NUMBER;
                 }
            }
            print "NO MATCH FOR: 0$num\n";
}

__DATA__
0201    Essen, Ruhr
0202    Wuppertal
0203    Duisburg
02041    Bottrop
02043    Gladbeck Westf
0208    Muelheim a.d. Ruhr
0208    Oberhausen Rheinl
212    Solingen
02129    Haan Rheinl
030    Berlin
03301    Oranienburg
03302    Hennigsdorf
03303    Birkenwerder
03304    Velten
033051    Nassenheide
033053    Zehlendorf Kr Oberhav
033054    Liebenwalde
033055    Kremmen
033056    Muehlenbeck Kr Oberhav
03306    Gransee
03307    Zehdenick

[download]

This prints: 

0204112345    = 02041-12345   in sunny Bottrop
03304123456   = 03304-123456  in sunny Velten
NO MATCH FOR: 03145666
0301234567    =   030-1234567 in sunny Berlin

[download]
Well, 'sunny' is just wishful thinking...

Update: removed, added

In reply to Re: (german) region code detection - request for thoughts by Perlbotics
in thread (german) region code detection - request for thoughts by Skeeve

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