Rewrite the regex as a straight loop

Author: davidwengier

src/Compiler/Microsoft.AspNetCore.Razor.Language/test/HtmlConventionsTest.cs

@@ -21,7 +21,7 @@ public static TheoryData HtmlConversionData
                     { "CAPSOnOUTSIDE", "caps-on-outside" },
                     { "ALLCAPS", "allcaps" },
                     { "One1Two2Three3", "one1-two2-three3" },
-                    { "ONE1TWO2THREE3", "one1two2three3" },
+                    { "ONE1TWO2THREE3", "one1-two2-three3" },
                     { "First_Second_ThirdHi", "first_second_third-hi" }
                 };
         }
@@ -35,6 +35,6 @@ public void ToHtmlCase_ReturnsExpectedConversions(string input, string expectedO
         var output = HtmlConventions.ToHtmlCase(input);
 
         // Assert
-        Assert.Equal(output, expectedOutput);
+        Assert.Equal(expectedOutput, output);
     }
 }

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/HtmlConventions.cs

@@ -4,26 +4,14 @@
 #nullable disable
 
 using System;
-using System.Text.RegularExpressions;
+using Microsoft.AspNetCore.Razor.PooledObjects;
 
 namespace Microsoft.AspNetCore.Razor.Language;
 
 public static class HtmlConventions
 {
-    private const string HtmlCaseRegexReplacement = "-$1$2";
     private static readonly char[] InvalidNonWhitespaceHtmlCharacters =
-        new[] { '@', '!', '<', '/', '?', '[', '>', ']', '=', '"', '\'', '*' };
-
-    // This matches the following AFTER the start of the input string (MATCH).
-    // Any letter/number followed by an uppercase letter then lowercase letter: 1(Aa), a(Aa), A(Aa)
-    // Any lowercase letter followed by an uppercase letter: a(A)
-    // Each match is then prefixed by a "-" via the ToHtmlCase method.
-    private static readonly Regex HtmlCaseRegex =
-        new Regex(
-            "(?<!^)((?<=[a-zA-Z0-9])[A-Z][a-z])|((?<=[a-z])[A-Z])",
-            RegexOptions.None,
-            TimeSpan.FromMilliseconds(500));
-
+        ['@', '!', '<', '/', '?', '[', '>', ']', '=', '"', '\'', '*'];
 
     internal static bool IsInvalidNonWhitespaceHtmlCharacters(char testChar)
     {
@@ -52,6 +40,47 @@ internal static bool IsInvalidNonWhitespaceHtmlCharacters(char testChar)
     /// </example>
     public static string ToHtmlCase(string name)
     {
-        return HtmlCaseRegex.Replace(name, HtmlCaseRegexReplacement).ToLowerInvariant();
+        if (string.IsNullOrEmpty(name))
+        {
+            return name;
+        }
+
+        var input = name.AsSpan();
+
+        using var _ = StringBuilderPool.GetPooledObject(out var result);
+        result.SetCapacityIfLarger(input.Length + 5);
+
+        // It's slightly faster to use a foreach and index manually, than to use a for 🤷‍
+        var i = 0;
+        foreach (var c in name)
+        {
+            if (char.IsUpper(c))
+            {
+                // Insert hyphen if:
+                // - Not the first character, and
+                // - Previous is lowercase or digit, or
+                // - Previous is uppercase and next is lowercase (e.g. CAPSOn → caps-on)
+                if (i > 0)
+                {
+                    var prev = input[i - 1];
+                    var prevIsLowerOrDigit = char.IsLower(prev) || char.IsDigit(prev);
+                    var prevIsUpper = char.IsUpper(prev);
+                    var nextIsLower = (i + 1 < input.Length) && char.IsLower(input[i + 1]);
+                    if (prevIsLowerOrDigit || (prevIsUpper && nextIsLower))
+                    {
+                        result.Append('-');
+                    }
+                }
+                result.Append(char.ToLowerInvariant(c));
+            }
+            else
+            {
+                result.Append(c);
+            }
+
+            i++;
+        }
+
+        return result.ToString();
     }
 }