#! perl -slw
use strict;

my @fields = (
	'1234',
	'01234567890',
	'abcd1234xy',
	'012 345 6789',
	'+44(0)1234 567890',
);

for (@fields) {
    (my $new = $_) =~ s[\D+][]g;
    print "Old:'$_' New:'$new'??" 
        if length $new != 4 and length $new != 10;
}