Regex for matching a US phone number

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I have written my first regex:

static void Main(string[] args)
    string phoneNumber= Console.ReadLine();

    Regex pattern = new Regex("(1-)?\p{N}{3}-\p{N}{3}-\p{N}{4}\b");

    MatchCollection collection = pattern.Matches(phoneNumber);

    Console.WriteLine(phoneNumber + " is " + (collection.Count == 1 && collection[0].ToString() == phoneNumber ? "" : "not ") + "a valid US number.");

It needs to match the entire input string, and it can work for any US phone number with or without country code “1-“. It can most likely be improved; any suggestions?


Use of Regex Objects

If your intent is merely to validate that the input is a phone number or not, rather than trying to capture anything, you can use the static Regex.IsMatch function:

var isPhoneNumber = Regex.IsMatch(phoneNumber, "(1-)?\p{N}{3}-\p{N}{3}-\p{N}{4}\b");

That saves having to deal with creating a new object and deal with a MatchCollection

Regex Pattern

There are a few things I would change:

  • There is a simple character class for matching digits 0-9: d. p{N} may be more appropriate if you need to match the entire Unicode number group rather than simple digits, but then I would not expect to see the explicit 1 at the start.
  • b matches backspace characters, which seems a little odd in a phone number. I would omit it. If your phone number strings happen to have backspace characters at the end, it will still match, but I would not require it be there.
  • Given the backslashes necessary for character escapes, I would prefix your string with the @ symbol for readability

Also, as @miasbeck points out, if you want to match the entire input string, you need to prefix the pattern with ^ and suffix it with $. They indicate that the match must start at the beginning and end of the string, respectively.

The end result would be something like the following:


Additionally, since there is no requirement that an area code is used for local numbers, so you may want to make it optional as well:


(note: The MSDN page on Regular Expression Language is always a good reference to check on when coming up with regex patterns in .NET code.)

String Formatting

Rather than concatenating the strings together, I would use a format string:

const string FMT = "{0} is {1} a valid US number.";

Additionally, instead of using "", I always tend towards string.Empty, as it is much more explicit about intent. "" could easily be a mistake, but string.Empty is clear that you really do want an empty string.

You can then output your result as follows:

Console.WriteLine(FMT, phoneNumber, (isPhoneNumber ? string.Empty : "not "));

Bringing it Together

Taking all the suggestions together, you get the following:

static void Main(string[] args)
   const string FMT = "{0} is {1} a valid US number.";
   var phoneNumber= Console.ReadLine();
   var isPhoneNumber = Regex.IsMatch(phoneNumber, @"^((1-)?d{3}-)?d{3}-d{4}$");
   Console.WriteLine(FMT, phoneNumber, (isPhoneNumber ? string.Empty : "not "));

(note: I switch back and forth between C# and F# , so I use var more often than most – feel free to keep the explicit type declarations if you really want)

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