Detecting plurals

PluralisationHelper.cs

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

namespace LanguageUtils
{
    public static class PluralisationHelper
    {
        private static readonly Dictionary<string, string> KnownCommonPluralsDictionary = new Dictionary<string, string>
        {
            {"children", "child"},
            {"people", "person"}
        };

        /// <summary>
        ///     This is far from perfect but detects the majority of english language plurals
        ///     See https://en.wikipedia.org/wiki/English_plurals#Irregular_plurals and
        ///     and http://grammar.yourdictionary.com/grammar-rules-and-tips/irregular-plurals.html
        ///     for useful example cases.
        /// </summary>
        public static bool IsPlural(string plural, string singular)
        {
            // dogs => dog
            if (plural.EndsWith("s") && singular == ReplaceEnd(plural, "s"))
                return true;

            // babies => baby
            if (singular.EndsWith("y") && plural == ReplaceEnd(singular, "y", "ies"))
                return true;

            // catches => catch
            if (plural.EndsWith("es") && singular == ReplaceEnd(plural, "es"))
                return true;

            // people => person and other "one off" edge cases
            if (KnownCommonPluralsDictionary.ContainsKey(plural) && KnownCommonPluralsDictionary[plural] == singular)
                return true;

            //series => series
            if (plural.EndsWith("ies") && plural == singular)
                return true;

            //statuses => status
            if (plural.EndsWith("uses") && singular == ReplaceEnd(plural, "es"))
                return true;

            //synopses => synopsis
            if (plural.EndsWith("ses") && singular == ReplaceEnd(plural, "es", "is"))
                return true;

            //wives => wife
            if (singular.EndsWith("fe") && plural == ReplaceEnd(singular, "fe", "ves"))
                return true;

            //halves => half
            if (singular.EndsWith("f") && plural == ReplaceEnd(singular, "f", "ves"))
                return true;

            //women => woman
            if (singular.EndsWith("man") && plural == ReplaceEnd(singular, "man", "men"))
                return true;

            //vertices => vertex
            if (singular.EndsWith("ex") && plural == ReplaceEnd(singular, "ex", "ices"))
                return true;

            //matricies => matrix
            if (singular.EndsWith("ix") && plural == ReplaceEnd(singular, "ix", "ices"))
                return true;

            //indicies => index
            if (singular.EndsWith("ex") && plural == ReplaceEnd(singular, "ex", "ices"))
                return true;

            //axes => axis
            if (singular.EndsWith("is") && plural == ReplaceEnd(singular, "is", "es"))
                return true;

            //medium => media
            if (singular.EndsWith("um") && plural == ReplaceEnd(singular, "um", "a"))
                return true;

            return false;
        }

        private static string ReplaceEnd(string word, string trimString, string replacement = "")
        {
            return Regex.Replace(word, $"{trimString}$", replacement);
        }
    }
}

Test.cs

using System.Collections.Generic;
using NUnit.Framework;

namespace LanguageUtils
{
    [TestFixture]
    public class PluralisationHelpertests 
    {
        [TestCase("Synopses", "Synopsis", true)]
        [TestCase("Men", "Man", true)]
        [TestCase("Women", "Woman", true)]
        [TestCase("People", "Person", true)]
        [TestCase("Children", "Child", true)]
        [TestCase("Synopses", "NotSynopses", false)]
        [TestCase("Synopses", "SynopsesNot", false)]
        [TestCase("Codes", "Code", true)]
        [TestCase("Babies", "Baby", true)]
        [TestCase("Boxes", "Box", true)]
        [TestCase("Wives", "Wife", true)]
        [TestCase("Tomatoes", "Tomato", true)]
        [TestCase("Catches", "Catch", true)]
        [TestCase("Statuses", "Status", true)]
        [TestCase("Urls", "Url", true)]
        [TestCase("Ids", "Id", true)]
        [TestCase("Types", "Type", true)]
        [TestCase("Types", "TypeNote", false)]
        [TestCase("Series", "Series", true)]
        [TestCase("indexes", "index", true)]
        [TestCase("indices", "index", true)]
        [TestCase("media", "medium", true)]
        [TestCase("viruses", "virus", true)]
        [TestCase("halves", "half", true)]
        public void IdentifyCommonPluralsCorrectly(string plural, string singular, bool isPlural)
        {
            var actual = PluralisationHelper.IsPlural(plural,singular);

            Assert.AreEqual(isPlural, actual, $"{plural} is{(isPlural ? "" : " not")} a plural of {singular}");
        }
      }
  }