--- Perl sample codes -----

my (@list) = ( "12", "123", "1234", "12345",
"123456", "1234567", "234", "2345", "23456" );

foreach $line (@list)
{
$line =~ s/(?<=\w)(?=(?:\w\w\w)+\b)/-/g;
print "$line<br>";
}

##</code><code>##

--- results of above code
12
123
1-234
12-345
123-456
1-234-567
234
2-345
23-456

##</code><code>##

123-4
123-45
123-456
123-456-7