From 3e9a65a1ac79e2c0830bff7fe2945f38cd34cf16 Mon Sep 17 00:00:00 2001
From: Will Ray <willray@propertyme.com>
Date: Thu, 29 Jan 2026 10:50:13 +1100
Subject: [PATCH] Preserve ALL-CAPS pluralization for multi-letter inputs; add
 test

---
 src/Humanizer.Tests/InflectorTests.cs   |  7 ++++++
 src/Humanizer/Inflections/Vocabulary.cs | 33 ++++++++++++++++++++++---
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/src/Humanizer.Tests/InflectorTests.cs b/src/Humanizer.Tests/InflectorTests.cs
index 1c6bed24e..84e299b20 100644
--- a/src/Humanizer.Tests/InflectorTests.cs
+++ b/src/Humanizer.Tests/InflectorTests.cs
@@ -40,6 +40,13 @@ public void PluralizeWordsWithUnknownPlurality(string singular, string plural)
         Assert.Equal(plural, singular.Pluralize(false));
     }
 
+    [Fact]
+    public void Pluralize_Preserves_AllCaps_Suffix()
+    {
+        Assert.Equal("SINGULAR TYPE NAMES", "SINGULAR TYPE NAME".Pluralize());
+        Assert.Equal("SINGULAR TYPE NAMES", "singular type name".Humanize(LetterCasing.AllCaps).Pluralize());
+    }
+
     [Theory]
     [ClassData(typeof(PluralTestSource))]
     public void Singularize(string singular, string plural) =>
diff --git a/src/Humanizer/Inflections/Vocabulary.cs b/src/Humanizer/Inflections/Vocabulary.cs
index 08f516f9e..47496a0e7 100644
--- a/src/Humanizer/Inflections/Vocabulary.cs
+++ b/src/Humanizer/Inflections/Vocabulary.cs
@@ -195,9 +195,36 @@ public void AddSingular(string rule, string replacement) =>
     bool IsUncountable(string word) =>
         uncountables.Contains(word);
 
-    static string MatchUpperCase(string word, string replacement) =>
-        char.IsUpper(word[0]) &&
-        char.IsLower(replacement[0]) ? StringHumanizeExtensions.Concat(char.ToUpper(replacement[0]), replacement.AsSpan(1)) : replacement;
+    static string MatchUpperCase(string word, string replacement)
+    {
+        var lettersCount = 0;
+        var allLettersUpper = true;
+        foreach (var c in word)
+        {
+            if (char.IsLetter(c))
+            {
+                lettersCount++;
+                if (!char.IsUpper(c))
+                {
+                    allLettersUpper = false;
+                    break;
+                }
+            }
+        }
+
+        // If the input contains multiple letters and all of them are uppercase
+        // (e.g. an ALL CAPS phrase or acronym of length > 1), return the replacement
+        // fully uppercased so the plural suffix matches the input casing.
+        if (lettersCount > 1 && allLettersUpper)
+        {
+            return replacement.ToUpperInvariant();
+        }
+
+        return char.IsUpper(word[0]) && char.IsLower(replacement[0])
+            ? StringHumanizeExtensions.Concat(char.ToUpper(replacement[0]), replacement.AsSpan(1))
+            : replacement;
+    }
+
 
     /// <summary>
     /// If the word is the letter s, singular or plural, return the letter s singular