exercise: PhoneNumber
version: 3
plan: 16
subs: clean_number
tests: |-
  foreach my $subcases (@{$C_DATA->{cases}}) {
    is $subs{clean_number}->($_->{input}{phrase}), $_->{expected}, $_->{description} foreach @{$subcases->{cases}};
  }

example: |
  sub clean_number {
    my ($number) = @_;
    $number =~ s/\D//g;
    return $number =~ /^ 1? ( (?: [2-9] \d{2} ){2} \d{4} ) $/x ? $1 : undef;
  }