NANPATelephoneFormatDetection

North American Numbering Plan Administrator

North American Numbering Plan The formatting rules:

Easily Recognizable Codes When the second and third digits of an area code are the same, that code is called an easily recognizable code (ERC). ERCs designate special services; e.g., 888 for toll-free service.
N11 These 8 ERCs, called service codes, are not used as area codes.
N9X The 80 codes in this format, called expansion codes, have been reserved for use during the period when the current 10-digit NANP number format undergoes expansion.
37X and 96X Two blocks of 10 codes each have been set aside by the INC for unanticipated purposes where it may be important to have a full range of 10 contiguous codes available.
(?<ERC>^([2-9](11|22|33|44|55|66|77|88|99)$))|
- (
  - (?<NonGeoSvc>^(5(00|21|22|23|24|25|26|27|28|29|33|44|66|77|88))|
  - (?<CAservices>^(6(00|22))|
  - (?<InterExch>^(700))|
  - (?<USgovt>^(710))
  - (?<TollFree>^(8(00|33|44|55|66|77|88)))|
  - (?<Premium>^(900)
- )(?<SpecExchange>([2-9][0-9][0-9])-(?<SpecNumber>[0-9]{4}))
(?<ReservedAreaCode>([2-9]9[0-9])|((37|96)[0-9]))-[2-9][0-9][0-9]-[0-9]{4}$
(?<InfoAreaCode>^([2-9][0-8][0-9])-555-(?<InfoNum>(1212 4334)$)|
(?<EntAreaCode>^[2-9][0-8][0-9])-555-(?<EntNumber>(01[0-9][0-9]))$|
(?<AreaCode>([2-9][0-8][0-9]))-(?<Exchange>([2-9][0-9][0-9]))-(?<Number>([0-9]{4}))$

The rules:

Numbers must be in the following format: ddd, or ddd-ddd-dddd. No parentheses are allowed.
If ERC is a group that is matched and the length is greater than 3, the number is invalid.