How to validate a (country specific) phone number

Question

A valid phone number contains:

• Less than 9 characters
• A "+" at the start
• Only digits.

I\'m trying to use regular expre

Answer 1

This solution validates every test criteria for validating a phone number, it also leverages from the Regex API. Criteria includes spacing, any non numeric values, area codes (which you specify), number of values (digits) the phone number should have, and also includes error messaging as well as phone number old and new state.

Here is the source code:

public class PhoneNumberValidator
{
    public string ErrorMessage { get; set; }
    public int PhoneNumberDigits { get; set; }
    public string CachedPhoneNumber { get; set; }

    private Dictionary VaildAreaCodes()
    {
        return new Dictionary
        {
            [3] = "0",
            [4] = "27"
        };
    }

    private bool IsInteger(string value)
    {
        return int.TryParse(value, out int result);
    }

    private string GetConsecutiveCharsInPhoneNumberStr(string phoneNumber)
    {
        switch (PhoneNumberDigits)
        {
            case 0:
            case 10:
                PhoneNumberDigits = 10;
                return phoneNumber.Substring(phoneNumber.Length - 7);

            case 11:
                return phoneNumber.Substring(phoneNumber.Length - 8);

            default:
                return string.Empty;
        }
    }

    private bool IsValidAreaCode(ref string phoneNumber, string areaCode)
    {
        if (!IsInteger(areaCode))
        {
            ErrorMessage = "Area code characters of Phone Number value should only contain integers.";
            return false;
        }

        var areaCodeLength = areaCode.Length;
        var invalidAreaCodeMessage = "Phone Number value contains invalid area code.";
        switch (areaCodeLength)
        {
            case 2:
                phoneNumber = string.Concat("0", phoneNumber);
                return true;

            case 3:
                if (!areaCode.StartsWith(VaildAreaCodes[3]))
                    ErrorMessage = invalidAreaCodeMessage;
                return string.IsNullOrWhiteSpace(ErrorMessage) ? true : false;

            case 4:
                if (areaCode.StartsWith(VaildAreaCodes[4]))
                {
                    phoneNumber = string.Concat("0", phoneNumber.Remove(0, 2)); // replace first two charaters with zero
                    return true;
                }                    
                ErrorMessage = invalidAreaCodeMessage;
                return false;                

            default:
                ErrorMessage = invalidAreaCodeMessage;
                return false;
        }
    }   

    public bool IsValidPhoneNumber(ref string phoneNumber)
    {
        CachedPhoneNumber = phoneNumber;

        if (string.IsNullOrWhiteSpace(phoneNumber))
        {
            ErrorMessage = "Phone Number value should not be equivalent to null.";
            return false;
        }

        phoneNumber = Regex.Replace(phoneNumber, " {2,}", string.Empty); // remove all whitespaces
        phoneNumber = Regex.Replace(phoneNumber, "[^0-9]", string.Empty); // remove all non numeric characters

        var lastConsecutiveCharsInPhoneNumberStr = GetConsecutiveCharsInPhoneNumberStr(phoneNumber);

        if (string.IsNullOrWhiteSpace(lastConsecutiveCharsInPhoneNumberStr))
        {
            ErrorMessage = "Phone Number value not supported.";
            return false;
        }

        if (!IsInteger(lastConsecutiveCharsInPhoneNumberStr))
        {
            ErrorMessage = "Last consecutive characters of Phone Number value should only contain integers.";
            return false;
        }

        var phoneNumberAreaCode = phoneNumber.Replace(lastConsecutiveCharsInPhoneNumberStr, "");

        if (!IsValidAreaCode(ref phoneNumber, phoneNumberAreaCode))
        {
            return false;
        }            

        if (phoneNumber.Length != PhoneNumberDigits)
        {
            ErrorMessage = string.Format("Phone Number value should contain {0} characters instead of {1} characters.", PhoneNumberDigits, phoneNumber.Length);
            return false;
        }

        return true;
    }
}

The solution is highly configurable and may be used for any digits phone number as well as area code.