#! perl -slw
use strict;

my $re = qr[^\d{3}-\d{3}$];
my $pat = '111-111';

my @inputs = (
	'',
	'a',
	'1',
	'12',
	'12x',
	'123',
	'1234',
	'123-',
	'123x',
	'123-4',
	'123-x',
	'123-456',
	'123-4567'
);

for my $input ( @inputs ) {
	print "$input failed" and next if length $input > length $pat;
	my $test = $input . substr( $pat, length( $input ) );
	print $input, $test =~ $re ? ' passed' : ' failed';
}

__END__
P:\test>471541
 passed
a failed
1 passed
12 passed
12x failed
123 passed
1234 failed
123- passed
123x failed
123-4 passed
123-x failed
123-456 passed
123-4567 failed

##</code><code>##

#! perl -slw
use strict;

my @re = (
	'^$',
	'^\d{0,3}$',
	'^\d{3}-$',
	'^\d{3}-\d{0,3}$',
	'^\d{3}-\d{3}$',
);

my $re = join '|', reverse @re;

$re =  qr[$re];

my @inputs = (
	'',
	'a',
	'1',
	'12',
	'12x',
	'123',
	'1234',
	'123-',
	'123x',
	'123-4',
	'123-x',
	'123-456',
	'123-4567'
);

for my $input ( @inputs ) {
	print $input, $input =~ $re ? ' passed ' : ' failed ', 'the alternation';
	
	for my $reg ( @re ) {
		if( $input =~ $reg ) {
			print "$input passed";
			last;
		} else {
			print "$input failed";
		}
	}
}

__END__
P:\test>471541-2
 passed the alternation
 passed
a failed the alternation
a failed
a failed
a failed
a failed
a failed
1 passed the alternation
1 failed
1 passed
12 passed the alternation
12 failed
12 passed
12x failed the alternation
12x failed
12x failed
12x failed
12x failed
12x failed
123 passed the alternation
123 failed
123 passed
1234 failed the alternation
1234 failed
1234 failed
1234 failed
1234 failed
1234 failed
123- passed the alternation
123- failed
123- failed
123- passed
123x failed the alternation
123x failed
123x failed
123x failed
123x failed
123x failed
123-4 passed the alternation
123-4 failed
123-4 failed
123-4 failed
123-4 passed
123-x failed the alternation
123-x failed
123-x failed
123-x failed
123-x failed
123-x failed
123-456 passed the alternation
123-456 failed
123-456 failed
123-456 failed
123-456 passed
123-4567 failed the alternation
123-4567 failed
123-4567 failed
123-4567 failed
123-4567 failed
123-4567 failed