Fix bug in "Change casing" - thx Hlsgs :)

Fix #8903

src/Test/Core/StringExtensionsTest.cs

@@ -403,6 +403,22 @@ namespace Test.Core
             Assert.AreEqual("<i>How Are You?</i>", res);
         }
 
+        [TestMethod]
+        public void ToLowercaseButKeepTags1()
+        {
+            var input = "<i>HOW ARE YOU?</i>";
+            var res = input.ToLowercaseButKeepTags();
+            Assert.AreEqual("<i>how are you?</i>", res);
+        }
+
+        [TestMethod]
+        public void ToLowercaseButKeepTags2()
+        {
+            var input = "{\\c&H0000FF&}Red";
+            var res = input.ToLowercaseButKeepTags();
+            Assert.AreEqual("{\\c&H0000FF&}red", res);
+        }
+
         [TestMethod]
         public void HasSentenceEndingCultureNeutralTest()
         {

src/libse/Common/FixCasing.cs

@@ -250,7 +250,7 @@ namespace Nikse.SubtitleEdit.Core.Common
             var text = original;
             if (FixNormal)
             {
-                if (FixNormalOnlyAllUppercase && text != text.ToUpper(subtitleCulture))
+                if (FixNormalOnlyAllUppercase && HtmlUtil.RemoveHtmlTags(text, true) != HtmlUtil.RemoveHtmlTags(text, true).ToUpper(subtitleCulture))
                 {
                     return text;
                 }
@@ -258,7 +258,7 @@ namespace Nikse.SubtitleEdit.Core.Common
                 if (text.Length > 1)
                 {
                     // first all to lower
-                    text = text.ToLower(subtitleCulture).Trim();
+                    text = text.ToLowercaseButKeepTags().Trim();
                     text = text.FixExtraSpaces();
                     var st = new StrippableText(text);
                     st.FixCasing(nameList, false, true, true, lastLine, millisecondsFromLast); // fix all casing but names (that's a separate option)

src/libse/Common/StringExtensions.cs

@@ -500,6 +500,19 @@ namespace Nikse.SubtitleEdit.Core.Common
             return RestoreSavedTags(properCaseText, tags);
         }
 
+        public static string ToLowercaseButKeepTags(this string input)
+        {
+            if (string.IsNullOrWhiteSpace(input))
+            {
+                return input;
+            }
+
+            var sb = new StringBuilder();
+            var tags = RemoveAndSaveTags(input, sb, new SubRip());
+            var lowercaseText = sb.ToString().ToLowerInvariant();
+            return RestoreSavedTags(lowercaseText, tags);
+        }
+
         public static string ToggleCasing(this string input, SubtitleFormat format, string overrideFromStringInit = null)
         {
             if (string.IsNullOrWhiteSpace(input))