C# string SafeFormat method - used as a wrapper to string.Format() and passing Undefined to any missing arguments

StringUtils.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

public class StringUtils
{
    public static string SafeFormat(string format, params object[] parameters)
    {
        const int MAX_NUMBER_OF_UNPASSED_PARAMETERS = 50;
        const string UNDEFINED_PARAMETER_VALUE = "Undefined";

        List<object> parametersList = new List<object>(parameters);
        List<int> parameterIndexes = new List<int>();

        // Use negative look behind to skip double {
        // Capture the indexes digits after { in groups named "ParameterIndex"
        const string RE_PATTERN = @"(?<!{){(?<ParameterIndex>\d+)";
        Regex re = new Regex(RE_PATTERN);
        MatchCollection matches = re.Matches(format);
        foreach (Match match in matches)
            parameterIndexes.Add(Convert.ToInt32(match.Groups["ParameterIndex"].Value));

        if (parameterIndexes.Any())
        {
            int maxIndex = parameterIndexes.Max();

            if (maxIndex >= parametersList.Count)
            {
                int parametersToAdd = (maxIndex - parametersList.Count) + 1;
                if (parametersToAdd > MAX_NUMBER_OF_UNPASSED_PARAMETERS)
                    throw new FormatException($"Invalid format {format}.It would require {parametersToAdd} more arguments to pass");
                parametersList.AddRange(Enumerable.Repeat(UNDEFINED_PARAMETER_VALUE, parametersToAdd));
            }
        }

        return string.Format(format, parametersList.ToArray());
    }

}

StringUtilsTests.cs

using Microsoft.VisualStudio.TestTools.UnitTesting;
using System;

[TestClass]
public class StringUtilsTests
{
    [TestMethod]
    public void StringUtilsSafeFormat_FormatWithoutPlaceholders_Succeeds()
    {
        string template = "num";
        string result = StringUtils.SafeFormat(template);

        Assert.IsNotNull(result);
        Assert.AreEqual("num", result);
    }

    [TestMethod]
    public void StringUtilsSafeFormat_WithCorrectNumberOfArguments_Succeeds()
    {
        string template = "num {0}";
        string result = StringUtils.SafeFormat(template, 1);

        Assert.IsNotNull(result);
        Assert.AreEqual("num 1", result);
    }

    [TestMethod]
    public void StringUtilsSafeFormat_WithCorrectNumberOfArgumentsAndHexFormatting_Succeeds()
    {
        string template = "num {0:X}";
        string result = StringUtils.SafeFormat(template, 255);

        Assert.IsNotNull(result);
        Assert.AreEqual("num FF", result);
    }

    [TestMethod]
    public void StringUtilsSafeFormat_WithFewerNumberOfArguments_SucceedsAndFillsMissingArguments()
    {
        string template = "num {0}";
        string result = StringUtils.SafeFormat(template);

        Assert.IsNotNull(result);
        Assert.AreEqual("num Undefined", result);
    }

    [TestMethod]
    [ExpectedException(typeof(FormatException))]
    public void StringUtilsSafeFormat_WithTemplateThatWouldRequireManyArgumentsToBeAdded_Throws()
    {
        string template = "num {100}";
        string result = StringUtils.SafeFormat(template);
    }

    [TestMethod]
    public void StringUtilsSafeFormat_WithTemplateThatHasDoubleSquareBrackets_SucceedsAndDoesNoReplacement()
    {
        string template = "num {{1}}";
        string result = StringUtils.SafeFormat(template);

        Assert.IsNotNull(result);
        Assert.AreEqual("num {1}", result);
    }
}