Formatting Number with Regular Expression

[eluser]pickupman[/eluser]
Ah...you asked for 10 digit strings first time. I am not a regex wiz but a while loop may work easier as it would work for any length of string

Code:

$input = '1234567111890';
$output = '';

while(strlen($input) > 0){
    $output .= substr($input, 0, 3) . '-'; #take first 3 digits
    $input = substr($input, 3); #remove those 3 digits from string
}
$output = rtrim($output, '-'); #remove trailing -
var_dump($output);
unset($input); #tidy up

You may want to test if $input is numeric if you didn't want any alpha characters.