I'm trying to write a regular expression to validate US phone number of format (123)123-1234 or 123-123-1234 as true.

The easiest way to match both this (000)000-0000 is this

     ^\([0-9]{3}\)[0-9]{3}-[0-9]{4}$

and this 000-000-0000 is this

     ^[0-9]{3}-[0-9]{3}-[0-9]{4}$

and to use alternation ((...|...)): specify them as two mostly-separate options:

     ^(\([0-9]{3}\)|[0-9]{3}-)[0-9]{3}-[0-9]{4}$

By the way, when Americans put the area code in parentheses, we actually put a space after that; for example, I'd write (123) 123-1234, not (123)123-1234. So you might want to write:

     ^(\([0-9]{3}\) |[0-9]{3}-)[0-9]{3}-[0-9]{4}$

(Though it's probably best to explicitly demonstrate the format that you expect phone numbers to be in.)

Formula for field named [Mobile]:

=AND(IF(ISERROR(FIND("(", [Mobile],1)), FALSE, (FIND("(", [Mobile]) = 1)), IF(ISERROR(FIND(")", [Mobile],5)), FALSE, (FIND(")", [Mobile], 5) = 5)), IF(ISERROR(FIND(" ", [Mobile],6)), FALSE, (FIND(" ", [Mobile], 6) = 6)), IF(ISERROR(FIND("-", [Mobile],10)), FALSE, (FIND("-", [Mobile], 10) = 10)),IF(ISERROR(1*CONCATENATE(MID([Mobile], 2, 3), MID([Mobile], 7, 3), MID([Mobile], 11, 4))), FALSE, AND(1*CONCATENATE(MID([Mobile], 2, 3), MID([Mobile], 7, 3), MID([Mobile], 11, 4)) > 1000000000, 1*MID([Mobile], 2, 3) <> 911, 1*MID([Mobile], 7, 3) <> 911)))

Also see, https://thechriskent.com/2012/08/15/validate-phone-number-columns-in-sharepoint/