Validate phone numbers in C#

The plans in this file

Add the countries whose numbers you accept; the file and its tests below are generated again for them, in your browser.

• United Kingdom +44×
• United States +1×
• France +33×

1. The plans

Plans holds one entry per plan: its region (the ISO code), the country code, the trunk prefix dialled before a national number, the exit code dialled before an international one, and a pattern per line type. The patterns are the plans' own, anchored, in the order the file tries them: the special ranges first (premium rate, toll-free, shared cost, VoIP, personal, pager, UAN, voicemail), then fixed line, then mobile. A plan that names no single exit code gets an empty one, and calls out of it use the + form.

// Phone.cs: validate, normalise and dial phone numbers with the national numbering plans.
// Plans: GB +44, US +1, FR +33 (data as of 2026-10-25).
// Other plans: https://phonelookup.dev/guides/validate-phone-numbers-csharp?plans=gb,us,fr
// Each pattern matches a national significant number: digits only, no country code, no trunk prefix.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

public static class Phone
{
    /// <summary>One numbering plan: region, country code, trunk prefix, exit code and an anchored pattern per line type.</summary>
    public sealed record Plan(string Region, string Cc, string Trunk, string Exit, (string Name, Regex Pattern)[] Types);

    /// <summary>A number as read: E.164, the plan it belongs to, its line type, country code and national number.</summary>
    public sealed record PhoneNumber(string E164, string Region, string Type, string Cc, string Nsn);

    static (string, Regex) T(string name, string pattern) => (name, new Regex(pattern, RegexOptions.CultureInvariant));

    public static readonly Plan[] Plans =
    {
        new("GB", "44", "0", "00", new[]
        {
            T("premiumRate", @"\A(?:(?:8(?:4[2-5]|7[0-3])|9(?:[01]\d|8[2-49]))\d{7}|845464\d)\z"),
            T("tollFree", @"\A(?:80[08]\d{7}|800\d{6}|8001111)\z"),
            T("voip", @"\A(?:56\d{8})\z"),
            T("personalNumber", @"\A(?:70\d{8})\z"),
            T("pager", @"\A(?:76(?:464|652)\d{5}|76(?:0[0-28]|2[356]|34|4[01347]|5[49]|6[0-369]|77|8[14]|9[139])\d{6})\z"),
            T("uan", @"\A(?:(?:3[0347]|55)\d{8})\z"),
            T("fixedLine", @"\A(?:(?:1(?:1(?:3(?:[0-58]\d\d|73[0-5])|4(?:(?:[0-5]\d|70)\d|69[7-9])|(?:(?:5[0-26-9]|[78][0-49])\d|6(?:[0-4]\d|5[01]))\d)|(?:2(?:(?:0[024-9]|2[3-9]|3[3-79]|4[1-689]|[58][02-9]|6[0-47-9]|7[013-9]|9\d)\d|1(?:[0-7]\d|8[0-3]))|(?:3(?:0\d|1[0-8]|[25][02-9]|3[02-579]|[468][0-46-9]|7[1-35-79]|9[2-578])|4(?:0[03-9]|[137]\d|[28][02-57-9]|4[02-69]|5[0-8]|[69][0-79])|5(?:0[1-35-9]|[16]\d|2[024-9]|3[015689]|4[02-9]|5[03-9]|7[0-35-9]|8[0-468]|9[0-57-9])|6(?:0[034689]|1\d|2[0-35689]|[38][013-9]|4[1-467]|5[0-69]|6[13-9]|7[0-8]|9[0-24578])|7(?:0[0246-9]|2\d|3[0236-8]|4[03-9]|5[0-46-9]|6[013-9]|7[0-35-9]|8[024-9]|9[02-9])|8(?:0[35-9]|2[1-57-9]|3[02-578]|4[0-578]|5[124-9]|6[2-69]|7\d|8[02-9]|9[02569])|9(?:0[02-589]|[18]\d|2[02-689]|3[1-57-9]|4[2-9]|5[0-579]|6[2-47-9]|7[0-24578]|9[2-57]))\d)\d)|2(?:0[013478]|3[0189]|4[017]|8[0-46-9]|9[0-2])\d{3})\d{4}|1(?:2(?:0(?:46[1-4]|87[2-9])|545[1-79]|76(?:2\d|3[1-8]|6[1-6])|9(?:7(?:2[0-4]|3[2-5])|8(?:2[2-8]|7[0-47-9]|8[3-5])))|3(?:6(?:38[2-5]|47[23])|8(?:47[04-9]|64[0157-9]))|4(?:044[1-7]|20(?:2[23]|8\d)|6(?:0(?:30|5[2-57]|6[1-8]|7[2-8])|140)|8(?:052|87[1-3]))|5(?:2(?:4(?:3[2-79]|6\d)|76\d)|6(?:26[06-9]|686))|6(?:06(?:4\d|7[4-79])|295[5-7]|35[34]\d|47(?:24|61)|59(?:5[08]|6[67]|74)|9(?:55[0-4]|77[23]))|7(?:26(?:6[13-9]|7[0-7])|(?:442|688)\d|50(?:2[0-3]|[3-68]2|76))|8(?:27[56]\d|37(?:5[2-5]|8[239])|843[2-58])|9(?:0(?:0(?:6[1-8]|85)|52\d)|3583|4(?:66[1-8]|9(?:2[01]|81))|63(?:23|3[1-4])|9561))\d{3})\z"),
            T("mobile", @"\A(?:7(?:457[0-57-9]|700[01]|911[028])\d{5}|7(?:[1-3]\d\d|4(?:[0-46-9]\d|5[0-689])|5(?:0[0-8]|[13-9]\d|2[0-35-9])|7(?:0[1-9]|[1-7]\d|8[02-9]|9[0-689])|8(?:[014-9]\d|[23][0-8])|9(?:[024-9]\d|1[02-9]|3[0-689]))\d{6})\z"),
        }),
        new("US", "1", "1", "011", new[]
        {
            T("premiumRate", @"\A(?:900[2-9]\d{6})\z"),
            T("tollFree", @"\A(?:8(?:00|33|44|55|66|77|88)[2-9]\d{6})\z"),
            T("personalNumber", @"\A(?:52(?:3(?:[2-46-9][02-9]\d|5(?:[02-46-9]\d|5[0-46-9]))|4(?:[2-478][02-9]\d|5(?:[034]\d|2[024-9]|5[0-46-9])|6(?:0[1-9]|[2-9]\d)|9(?:[05-9]\d|2[0-5]|49)))\d{4}|52[34][2-9]1[02-9]\d{4}|5(?:00|2[125-9]|3[23]|44|66|77|88)[2-9]\d{6})\z"),
            T("fixedLine", @"\A(?:983[2-57-9]\d{6}|(?:2(?:0[1-35-9]|1[02-9]|2[03-57-9]|3[1459]|4[08]|5[1-46]|6[0279]|7[02469]|8[13])|3(?:0[1-57-9]|1[02-9]|2[013-79]|3[0-24679]|4[167]|5[0-3]|6[01349]|8[056])|4(?:0[124-9]|1[02-579]|2[3-5]|3[0245]|4[023578]|58|6[349]|7[02589]|8[04])|5(?:0[1-57-9]|1[0235-8]|20|3[0149]|4[01]|5[179]|6[1-47]|7[0-5]|8[0256])|6(?:0[1-35-9]|1[024-9]|2[03689]|3[016]|4[0156]|5[01679]|6[0-279]|78|8[0-269])|7(?:0[1-46-8]|1[2-9]|2[04-8]|3[0-2478]|4[0378]|5[47]|6[02359]|7[0-59]|8[156])|8(?:0[1-68]|1[02-8]|2[0168]|3[0-2589]|4[03578]|5[046-9]|6[02-5]|7[028])|9(?:0[1346-9]|1[02-9]|2[0589]|3[0146-8]|4[01357-9]|5[12469]|7[0-3589]|8[04-69]))[2-9]\d{6})\z"),
            T("mobile", @"\A(?:983[2-57-9]\d{6}|(?:2(?:0[1-35-9]|1[02-9]|2[03-57-9]|3[1459]|4[08]|5[1-46]|6[0279]|7[02469]|8[13])|3(?:0[1-57-9]|1[02-9]|2[013-79]|3[0-24679]|4[167]|5[0-3]|6[01349]|8[056])|4(?:0[124-9]|1[02-579]|2[3-5]|3[0245]|4[023578]|58|6[349]|7[02589]|8[04])|5(?:0[1-57-9]|1[0235-8]|20|3[0149]|4[01]|5[179]|6[1-47]|7[0-5]|8[0256])|6(?:0[1-35-9]|1[024-9]|2[03689]|3[016]|4[0156]|5[01679]|6[0-279]|78|8[0-269])|7(?:0[1-46-8]|1[2-9]|2[04-8]|3[0-2478]|4[0378]|5[47]|6[02359]|7[0-59]|8[156])|8(?:0[1-68]|1[02-8]|2[0168]|3[0-2589]|4[03578]|5[046-9]|6[02-5]|7[028])|9(?:0[1346-9]|1[02-9]|2[0589]|3[0146-8]|4[01357-9]|5[12469]|7[0-3589]|8[04-69]))[2-9]\d{6})\z"),
        }),
        new("FR", "33", "0", "00", new[]
        {
            T("premiumRate", @"\A(?:836(?:0[0-36-9]|[1-9]\d)\d{4}|8(?:1[2-9]|2[2-47-9]|3[0-57-9]|[569]\d|8[0-35-9])\d{6})\z"),
            T("tollFree", @"\A(?:80[0-5]\d{6})\z"),
            T("sharedCost", @"\A(?:8(?:1[01]|2[0156]|4[024]|84)\d{6})\z"),
            T("voip", @"\A(?:9\d{8})\z"),
            T("uan", @"\A(?:80[6-9]\d{6})\z"),
            T("fixedLine", @"\A(?:(?:26[013-9]|59[1-35-9])\d{6}|(?:[13]\d|2[0-57-9]|4[1-9]|5[0-8])\d{7})\z"),
            T("mobile", @"\A(?:(?:6(?:[0-24-8]\d|3[0-8]|9[589])|7[3-9]\d)\d{6})\z"),
        }),
    };

2. Reading a number

Parse keeps the digits and a leading plus. With the plus, or with the default region's exit code in front (00 from the United Kingdom, 011 from the United States), the first one to three digits are the country code: under ITU-T E.164 no code is the prefix of another, so at most one length names a plan, and the plans sharing a code (+1, +7, +44) are tried in turn. Without either, the number is national and is read in the default region's plan. The trunk prefix is tried stripped first, so “+44 (0)7400 123456” reads too. A number in both the fixed-line and the mobile ranges comes back as fixedLineOrMobile, which is every ordinary number in the +1 plans. Digits typed in other scripts are dropped with the punctuation; convert them first if your users type them. Anything no plan accepts gives null.

    /// <summary>
    /// Reads a number the way people type it: "+44 7400 123456", "07400 123456", "00 44 7400 123456".
    /// A number without + or an exit code is read in the plan of <paramref name="defaultRegion"/> (an ISO code such as "GB").
    /// Null when no plan in <see cref="Plans"/> accepts it.
    /// </summary>
    public static PhoneNumber? Parse(string text, string? defaultRegion = null)
    {
        var s = Regex.Replace(text, "[^0-9+]", "");
        var digits = s.Replace("+", "");
        if (digits.Length < 3 || digits.Length > 17) return null;
        if (s.StartsWith('+')) return International(digits);
        var home = Plans.FirstOrDefault(p => p.Region == defaultRegion);
        if (home is null) return null;
        if (home.Exit.Length > 0 && digits.StartsWith(home.Exit, StringComparison.Ordinal))
            return International(digits[home.Exit.Length..]);
        return Match(Plans.Where(p => p.Cc == home.Cc && !ReferenceEquals(p, home)).Prepend(home), digits);
    }

    // Country codes are prefix-free (ITU-T E.164): the first one to three digits that name a plan are the code.
    static PhoneNumber? International(string digits)
    {
        for (var size = 1; size <= Math.Min(3, digits.Length); size++)
        {
            var cc = digits[..size];
            var plans = Plans.Where(p => p.Cc == cc).ToList();
            if (plans.Count > 0) return Match(plans, digits[size..]);
        }
        return null;
    }

    // The trunk prefix is tried stripped first: "+44 (0)7400 123456" is common.
    static PhoneNumber? Match(IEnumerable<Plan> plans, string nsn)
    {
        foreach (var plan in plans)
        {
            var tries = plan.Trunk.Length > 0 && nsn.StartsWith(plan.Trunk, StringComparison.Ordinal)
                ? new[] { nsn[plan.Trunk.Length..], nsn }
                : new[] { nsn };
            foreach (var national in tries)
            {
                var type = TypeOf(plan, national);
                if (type is not null) return new PhoneNumber($"+{plan.Cc}{national}", plan.Region, type, plan.Cc, national);
            }
        }
        return null;
    }

    // The first matching type wins; a number in both the fixed-line and the mobile ranges is reported as such.
    static string? TypeOf(Plan plan, string nsn)
    {
        var hits = plan.Types.Where(t => t.Pattern.IsMatch(nsn)).Select(t => t.Name).ToList();
        if (hits.Count == 0) return null;
        return hits[0] == "fixedLine" && hits.Contains("mobile") ? "fixedLineOrMobile" : hits[0];
    }

3. Writing and dialling it

Store the E.164 form, a plus and digits only: one string per number, and every other format follows from it.ToTel writes the tel: URI for links (RFC 3966). DialFrom gives the digits a person dials from another country: the caller's exit code, the country code and the national number, or, between plans that share a country code, a national call with the trunk prefix. The dialling helper explains each case.

    /// <summary>The tel: URI (RFC 3966), for links.</summary>
    public static string ToTel(PhoneNumber n) => $"tel:{n.E164}";

    /// <summary>
    /// What to dial from a phone in <paramref name="fromRegion"/>. Inside one country code (the UK and Jersey, the US and
    /// Canada) the call is dialled like a national one, with the trunk prefix; otherwise the exit code, the country code
    /// and the national number. The + form when the region is not in <see cref="Plans"/> or its plan names no exit code.
    /// </summary>
    public static string DialFrom(PhoneNumber n, string fromRegion)
    {
        var from = Plans.FirstOrDefault(p => p.Region == fromRegion);
        if (from is null) return n.E164;
        if (from.Cc == n.Cc) return from.Trunk + n.Nsn;
        return from.Exit.Length > 0 ? from.Exit + n.Cc + n.Nsn : n.E164;
    }
}

The tests

The cases are the plans' own example numbers, one per line type in international form; the first plan's mobile number again in national form, with “(0)”, and dialled with an exit code; then numbers no plan accepts, and three calls dialled. Their expected values are computed when the page is built, by the same logic in TypeScript. The generator behind this page is checked by running its output in xUnit, for these plans and for all 245 plans at once.

dotnet new xunit -o PhoneTests
# add Phone.cs and PhoneTests.cs to PhoneTests/
dotnet test PhoneTests

using Xunit;

public class PhoneTests
{
    [Theory]
    // typed, default region, E.164, line type
    [InlineData("+44 7400 123456", null, "+447400123456", "mobile")]
    [InlineData("+44 121 234 5678", null, "+441212345678", "fixedLine")]
    [InlineData("+44 800 123 4567", null, "+448001234567", "tollFree")]
    [InlineData("+44 56 1234 5678", null, "+445612345678", "voip")]
    [InlineData("+44 901 234 5678", null, "+449012345678", "premiumRate")]
    [InlineData("+44 70 1234 5678", null, "+447012345678", "personalNumber")]
    [InlineData("+44 76 4012 3456", null, "+447640123456", "pager")]
    [InlineData("+44 55 1234 5678", null, "+445512345678", "uan")]
    [InlineData("+1 201-555-0123", null, "+12015550123", "fixedLineOrMobile")]
    [InlineData("+1 800-234-5678", null, "+18002345678", "tollFree")]
    [InlineData("+1 900-234-5678", null, "+19002345678", "premiumRate")]
    [InlineData("+1 500-234-5678", null, "+15002345678", "personalNumber")]
    [InlineData("+33 6 12 34 56 78", null, "+33612345678", "mobile")]
    [InlineData("+33 1 23 45 67 89", null, "+33123456789", "fixedLine")]
    [InlineData("+33 801 23 45 67", null, "+33801234567", "tollFree")]
    [InlineData("+33 9 12 34 56 78", null, "+33912345678", "voip")]
    [InlineData("+33 884 01 23 45", null, "+33884012345", "sharedCost")]
    [InlineData("+33 891 12 34 56", null, "+33891123456", "premiumRate")]
    [InlineData("+33 806 12 34 56", null, "+33806123456", "uan")]
    [InlineData("07400 123456", "GB", "+447400123456", "mobile")]
    [InlineData("+44 (0)7400 123456", null, "+447400123456", "mobile")]
    [InlineData("00 1 201-555-0123", "GB", "+12015550123", "fixedLineOrMobile")]
    public void ReadsEachLineTypeOfEachPlan(string text, string? region, string e164, string type)
    {
        var number = Phone.Parse(text, region);
        Assert.NotNull(number);
        Assert.Equal(e164, number.E164);
        Assert.Equal(type, number.Type);
    }

    [Theory]
    [InlineData("12", "GB")]
    [InlineData("+44 7400 12345", null)]
    [InlineData("+49 1512 3456789", null)]
    [InlineData("07400 123456", null)]
    public void RejectsWhatNoPlanAccepts(string text, string? region) => Assert.Null(Phone.Parse(text, region));

    [Theory]
    [InlineData("+1 201-555-0123", "GB", "0012015550123")]
    [InlineData("+44 7400 123456", "US", "011447400123456")]
    [InlineData("+44 7400 123456", "GB", "07400123456")]
    public void Dials(string text, string from, string expected) => Assert.Equal(expected, Phone.DialFrom(Phone.Parse(text)!, from));

    [Fact]
    public void WritesTelUris() => Assert.Equal("tel:+447400123456", Phone.ToTel(Phone.Parse("+44 7400 123456")!));
}

What is specific to C#

• $ matches before a final newline in .NET too: the patterns are anchored with \A and \z.
• \d matches any Unicode decimal digit in .NET (Arabic-Indic, Devanagari, full-width). The module only ever hands the patterns ASCII digits; elsewhere, use [0-9] or RegexOptions.ECMAScript.
• Regex is immutable and thread-safe: the patterns are built once, in a static field. With .NET 7 or later, [GeneratedRegex] moves that work to compile time, one method per pattern.
• StartsWith on strings takes StringComparison.Ordinal: the default comparison is culture-sensitive.