❔ Help with simple regex

1234: 1.2.3.4
3453454: 3.4.5.3.4.5.4
343: 3.4.3
343455: 3.4.3.4.5.5

|                                   Method |               input |        Mean |     Error |    StdDev |   Gen0 | Allocated |
|----------------------------------------- |-------------------- |------------:|----------:|----------:|-------:|----------:|
|                 DottedNumbers_StringJoin |                 123 |    49.77 ns |  1.014 ns |  0.996 ns | 0.0081 |     136 B |
|                 DottedNumbers_StringJoin |                1337 |    62.06 ns |  1.207 ns |  1.129 ns | 0.0100 |     168 B |
|                 DottedNumbers_StringJoin |          0123456789 |   121.12 ns |  2.404 ns |  3.209 ns | 0.0200 |     336 B |
|                 DottedNumbers_StringJoin | 0123(...)6789 [100] |   986.00 ns | 19.298 ns | 27.677 ns | 0.1707 |    2856 B |
|  DottedNumbers_Regex_LookBehindLookAhead |                 123 |   162.02 ns |  2.115 ns |  1.875 ns | 0.0019 |      32 B |
|  DottedNumbers_Regex_LookBehindLookAhead |                1337 |   206.13 ns |  0.906 ns |  0.757 ns | 0.0024 |      40 B |
|  DottedNumbers_Regex_LookBehindLookAhead |          0123456789 |   466.24 ns |  2.322 ns |  2.058 ns | 0.0038 |      64 B |
|  DottedNumbers_Regex_LookBehindLookAhead | 0123(...)6789 [100] | 4,050.07 ns | 50.166 ns | 46.925 ns | 0.0229 |     424 B |
| DottedNumbers_Regex_SingleDigitLookAhead |                 123 |   217.83 ns |  0.724 ns |  0.677 ns | 0.0019 |      32 B |
| DottedNumbers_Regex_SingleDigitLookAhead |                1337 |   293.45 ns |  1.291 ns |  1.144 ns | 0.0024 |      40 B |
| DottedNumbers_Regex_SingleDigitLookAhead |          0123456789 |   684.73 ns | 12.974 ns | 12.742 ns | 0.0038 |      64 B |
| DottedNumbers_Regex_SingleDigitLookAhead | 0123(...)6789 [100] | 6,220.14 ns | 67.293 ns | 59.653 ns | 0.0229 |     424 B |

string.Join('.', "1234".ToCharArray())

1.2.3.4

1.2.3.4

Regex.Replace("1234", @"(\d)", "$1.")

1.2.3.4.

void F(string v) {
    Console.WriteLine($"{v}: {Regex.Replace(v, """(?<!^)(?=\d)""", ".")}");
}

F("1234");
F("3453454");
F("343");
F("343455");

Regex.Replace("1234", @"(\d)(?=\d)", "$1.")

[MemoryDiagnoser]
[Orderer(SummaryOrderPolicy.FastestToSlowest, MethodOrderPolicy.Alphabetical)]
public class Benchmarks
{
    [Benchmark]
    [Arguments("123")]
    [Arguments("1337")]
    [Arguments("0123456789")]
    [Arguments("0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789")]
    public string DottedNumbers_StringJoin(string input)
    {
        return string.Join('.', values: input);
    }

    [Benchmark]
    [Arguments("123")]
    [Arguments("1337")]
    [Arguments("0123456789")]
    [Arguments("0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789")]
    public string DottedNumbers_Regex_LookBehindLookAhead(string input)
    {
        return Regexes.GetLookBehindLookAheadRegex().Replace(input, ".");
    }

    [Benchmark]
    [Arguments("123")]
    [Arguments("1337")]
    [Arguments("0123456789")]
    [Arguments("0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789")]
    public string DottedNumbers_Regex_SingleDigitLookAhead(string input)
    {
        return Regexes.GetSingleDigitLookAheadRegex().Replace(input, "$1.");
    }
}

public static partial class Regexes
{
    [GeneratedRegex(@"(?<!^)(?=\d)", RegexOptions.Compiled)]
    public static partial Regex GetLookBehindLookAheadRegex();

    [GeneratedRegex(@"(\d)(?=\d)", RegexOptions.Compiled)]
    public static partial Regex GetSingleDigitLookAheadRegex();
}