Validate phone number with JavaScript

Question

I found this code in some website, and it works perfectly. It validates that the phone number is in one of these formats:
(123) 456-7890 or 123-

Answer 1

/^(()?\d{3}())?(-|\s)?\d{3}(-|\s)?\d{4}$/

The ? character signifies that the preceding group should be matched zero or one times. The group (-|\s) will match either a - or a | character.