Modified LineNumberRegex to recognize line numbers followed by colon. Line numbers followed by colon were not recognized as line numbers, but were instead treated as code segments, which caused the issue with indentation.

Author: AnnaVel

Rubberduck.SmartIndenter/AbsoluteCodeLine.cs

@@ -10,7 +10,7 @@ namespace Rubberduck.SmartIndenter
     internal class AbsoluteCodeLine
     {
         private const string StupidLineEnding = ": _";
-        private static readonly Regex LineNumberRegex = new Regex(@"^(?<number>(-?\d+)|(&H[0-9A-F]{1,8}))\s+(?<code>.*)", RegexOptions.ExplicitCapture);
+        private static readonly Regex LineNumberRegex = new Regex(@"^(?<number>(-?\d+)|(&H[0-9A-F]{1,8}))(?<separator>:)?\s+(?<code>.*)", RegexOptions.ExplicitCapture);
         private static readonly Regex EndOfLineCommentRegex = new Regex(@"^(?!(Rem\s)|('))(?<code>[^']*)(\s(?<comment>'.*))$", RegexOptions.ExplicitCapture);      
         private static readonly Regex ProcedureStartRegex = new Regex(@"^(Public\s|Private\s|Friend\s)?(Static\s)?(Sub|Function|Property\s(Let|Get|Set))\s");
         private static readonly Regex ProcedureStartIgnoreRegex = new Regex(@"^[LR]?Set\s|^Let\s|^(Public|Private)\sDeclare\s(Function|Sub)");
@@ -26,6 +26,7 @@ internal class AbsoluteCodeLine
 
         private readonly IIndenterSettings _settings;
         private int _lineNumber;
+        private bool _lineNumberSeparator;
         private bool _numbered;
         private string _code;
         private readonly bool _stupidLineEnding;
@@ -69,6 +70,7 @@ private void ExtractLineNumber()
                 {
                     _code = match.Groups["code"].Value;
                     _numbered = true;
+                    _lineNumberSeparator = match.Groups["separator"].Value != string.Empty;
                     var number = match.Groups["number"].Value;
                     if (!int.TryParse(number, out _lineNumber))
                     {
@@ -254,14 +256,15 @@ public string Indent(int indents, bool atProcStart, bool absolute = false)
             }
 
             var number = _numbered ? _lineNumber.ToString(CultureInfo.InvariantCulture) : string.Empty;
-            var gap = Math.Max((absolute ? indents : _settings.IndentSpaces * indents) - number.Length, number.Length > 0 ? 1 : 0);
+            var separator = _lineNumberSeparator ? ":" : string.Empty;
+            var gap = Math.Max((absolute ? indents : _settings.IndentSpaces * indents) - number.Length - separator.Length, number.Length + separator.Length > 0 ? 1 : 0);
             if (_settings.AlignDims && (IsDeclaration || IsDeclarationContinuation))
             {
                 AlignDims(gap);
             }
 
             var code = string.Join(": ", _segments);
-            code = string.Join(string.Empty, number, new string(' ', gap), code);
+            code = string.Join(string.Empty, number, separator, new string(' ', gap), code);
             if (string.IsNullOrEmpty(EndOfLineComment))
             {
                 return _escaper.UnescapeIndented(code + (_stupidLineEnding ? StupidLineEnding : string.Empty));

RubberduckTests/SmartIndenter/MiscAndCornerCaseTests.cs

@@ -298,6 +298,33 @@ public void HexLineNumbersWork()
             Assert.IsTrue(expected.SequenceEqual(actual));
         }
 
+        [TestMethod]
+        [TestCategory("Indenter")]
+        public void LineNumbersWithColonWork()
+        {
+            var code = new[]
+           {
+                "Private Sub Test()",
+                "5: If Foo Then",
+                "10: Debug.Print",
+                "15: End If",
+                "End Sub"
+            };
+
+            var expected = new[]
+            {
+                "Private Sub Test()",
+                "5:  If Foo Then",
+                "10:     Debug.Print",
+                "15: End If",
+                "End Sub"
+            };
+
+            var indenter = new Indenter(null, () => IndenterSettingsTests.GetMockIndenterSettings());
+            var actual = indenter.Indent(code);
+            Assert.IsTrue(expected.SequenceEqual(actual));
+        }
+
         [TestMethod]
         [TestCategory("Indenter")]
         public void LineNumberLongerThanIndentFallsBackToOneSpace()