C# Regex Phone Number Check

Question

I have the following to check if the phone number is in the following format (XXX) XXX-XXXX. The below code always return true. Not sure why.

   Match match = Regex.Match(input, @"((\(\d{3}\) ?)|(\d{3}-))?\d{3}-\d{4}");

    // Below code always return true
    if (match.Success) { ....}

Answer 1

The general complaint about regex patterns for phone numbers is that they require one to put in the truly optional characters as dashes and other items.

Why can't they be optional and have the pattern not care if they are there or not?

The below pattern makes dashes, periods and parenthesis optional for the user and focuses on the numbers as a result using named captures.

The pattern is commented (using the # and spans multiple lines) so use the Regex option IgnorePatternWhitespace unless one removes the comments. For that flag doesn't affect regex processing, it only allows for commenting of the pattern via the # character and line break .

string pattern = @"
^                  # From Beginning of line
(?:\(?)            # Match but don't capture optional (
(?<AreaCode>\d{3}) # 3 digit area code
(?:[\).\s]?)       # Optional ) or . or space
(?<Prefix>\d{3})   # Prefix
(?:[-\.\s]?)       # optional - or . or space
(?<Suffix>\d{4})   # Suffix
(?!\d)             # Fail if eleventh number found";

The above pattern just looks for 10 numbers and ignores any filler characters such as a ( or a dash - or a space or a tab or even a .. Examples are

(555)555-5555 (OK)
5555555555 (ok)
555 555 5555(ok)
555.555.5555 (ok)
55555555556 (not ok - match failure - too many digits)
123.456.789 (failure)

---

Different Variants of same pattern

Pattern without comments no longer need to use IgnorePatternWhiteSpace:

^(?:\(?)(?<AreaCode>\d{3})(?:[\).\s]?)(?<Prefix>\d{3})(?:[-\.\s]?)(?<Suffix>\d{4})(?!\d)

Pattern when not using Named Captures

^(?:\(?)(\d{3})(?:[\).\s]?)(\d{3})(?:[-\.\s]?)(\d{4})(?!\d)

Pattern if ExplicitCapture option is used

^\(?(?<AreaCode>\d{3})[\).\s]?(?<Prefix>\d{3})[-\.\s](?<Suffix>\d{4})(?!\d)

Answer 2

It doesn't always match, but it will match any string that contains three digits, followed by a hyphen, followed by four more digits. It will also match if there's something that looks like an area code on the front of that. So this is valid according to your regex:

%%%%%%%%%%%%%%(999)123-4567%%%%%%%%%%%%%%%%%

To validate that the string contains a phone number and nothing else, you need to add anchors at the beginning and end of the regex:

@"^((\(\d{3}\) ?)|(\d{3}-))?\d{3}-\d{4}$"