PHP Regular Expression telephone number matching-PHP source code

Ec (2); preg_match_all (& quot ;(? & Nbsp; (d {3 })? & Nbsp ;)? & Nbsp ;-? & Nbsp ;(? (1) & nbsp; [-s]) d {3}-d {4} x & quot;, & nbsp; & nbsp; & nb script ec (2); script

Preg_match_all ("/(? (D {3 })? )? -? (? (1) [-s]) d {3}-d {4}/x ",
"Call 555-1212 or (221)-(820)-555-1212", $ phones );
Print_r ($ phones [0]); echo"
";
Print_r ($ phones [1]);

The output result is:
Array ([0] => 555-1212 [1] => (820)-555-1212)
Array ([0] => [1] => 820)
Who can help explain what this regular expression means? The key is (? (D {3 })? )? (? (1) [-s.
Why is it possible to match (820) but not (221 )?

(? (D {3 })? )? (? (1) In [-s ])
(1) referenced (d {3 })
(? (1) [-s]) if (d {3}) matches successfully, [-s]
The prototype is (? A B | c). If a matches B, B is matched. Otherwise, c is matched.