Add support for Slack-flavored markdown

Author: Klikini

src/ReverseMarkdown/Cleaner.cs

@@ -6,6 +6,9 @@ namespace ReverseMarkdown
 {
     public static class Cleaner
     {
+        private static readonly Regex SlackBoldCleaner = new Regex(@"\*(\s\*)+");
+        private static readonly Regex SlackItalicCleaner = new Regex(@"_(\s_)+");
+        
         private static string CleanTagBorders(string content)
         {
             // content from some htl editors such as CKEditor emits newline and tab between tags, clean that up
@@ -28,5 +31,15 @@ public static string PreTidy(string content, bool removeComments)
 
             return content;
         }
+
+        public static string SlackTidy(string content)
+        {
+            // Slack's escaping rules depend on whether the key characters appear in
+            // next to word characters or not.
+            content = SlackBoldCleaner.Replace(content, "*");
+            content = SlackItalicCleaner.Replace(content, "_");
+            
+            return content;
+        }
     }
 }

src/ReverseMarkdown/Config.cs

@@ -8,7 +8,9 @@ public class Config
         public UnknownTagsOption UnknownTags { get; set; } = UnknownTagsOption.PassThrough;
 
         public bool GithubFlavored { get; set; } = false;
-        
+
+        public bool SlackFlavored { get; set; } = false;
+
         public bool SuppressDivNewlines { get; set; } = false;
 
         public bool RemoveComments { get; set; } = false;
@@ -31,10 +33,19 @@ public class Config
         public TableWithoutHeaderRowHandlingOption TableWithoutHeaderRowHandling { get; set; } =
             TableWithoutHeaderRowHandlingOption.Default;
 
+        private char _listBulletChar = '-';
+
         /// <summary>
         /// Option to set a different bullet character for un-ordered lists
         /// </summary>
-        public char ListBulletChar { get; set; } = '-';
+        /// <remarks>
+        /// This option is ignored when <see cref="SlackFlavored"/> is enabled.
+        /// </remarks>
+        public char ListBulletChar
+        {
+            get => SlackFlavored ? '•' : _listBulletChar;
+            set => _listBulletChar = value;
+        }
 
         /// <summary>
         /// Option to set a default GFM code block language if class based language markers are not available
@@ -52,14 +63,17 @@ public enum UnknownTagsOption
             /// Include the unknown tag completely into the result. That is, the tag along with the text will be left in output.
             /// </summary>
             PassThrough,
+
             /// <summary>
             ///  Drop the unknown tag and its content
             /// </summary>
             Drop,
+
             /// <summary>
             /// Ignore the unknown tag but try to convert its content
             /// </summary>
             Bypass,
+
             /// <summary>
             /// Raise an error to let you know
             /// </summary>
@@ -72,6 +86,7 @@ public enum TableWithoutHeaderRowHandlingOption
             /// By default, first row will be used as header row
             /// </summary>
             Default,
+
             /// <summary>
             /// An empty row will be added as the header row
             /// </summary>
@@ -90,10 +105,12 @@ public enum TableWithoutHeaderRowHandlingOption
         /// Determines whether url is allowed: WhitelistUriSchemes contains no elements or contains passed url.
         /// </summary>
         /// <param name="scheme">Scheme name without trailing colon</param>
-        internal bool IsSchemeWhitelisted(string scheme) {
+        internal bool IsSchemeWhitelisted(string scheme)
+        {
             if (scheme == null) throw new ArgumentNullException(nameof(scheme));
-            var isSchemeAllowed = WhitelistUriSchemes == null || WhitelistUriSchemes.Length == 0 || WhitelistUriSchemes.Contains(scheme, StringComparer.OrdinalIgnoreCase);
+            var isSchemeAllowed = WhitelistUriSchemes == null || WhitelistUriSchemes.Length == 0 ||
+                                  WhitelistUriSchemes.Contains(scheme, StringComparer.OrdinalIgnoreCase);
             return isSchemeAllowed;
         }
     }
-}
+}

src/ReverseMarkdown/Converter.cs

@@ -94,6 +94,11 @@ public virtual string Convert(string html)
             // cleanup multiple new lines
             result = Regex.Replace( result, @"(^\p{Zs}*(\r\n|\n)){2,}", Environment.NewLine, RegexOptions.Multiline);
 
+            if (Config.SlackFlavored)
+            {
+                result = Cleaner.SlackTidy(result);
+            }
+            
             return Config.CleanupUnnecessarySpaces ? result.Trim().FixMultipleNewlines() : result;
         }
 

src/ReverseMarkdown/Converters/Em.cs

@@ -30,7 +30,8 @@ public override string Convert(HtmlNode node)
                 ? " "
                 : "";
 
-            return content.EmphasizeContentWhitespaceGuard("*", spaceSuffix);
+            var emphasis = Converter.Config.SlackFlavored ? "_" : "*";
+            return content.EmphasizeContentWhitespaceGuard(emphasis, spaceSuffix);
         }
 
         private static bool AlreadyItalic(HtmlNode node)

src/ReverseMarkdown/Converters/Hr.cs

@@ -12,6 +12,11 @@ public Hr(Converter converter) : base(converter)
 
         public override string Convert(HtmlNode node)
         {
+            if (Converter.Config.SlackFlavored)
+            {
+                throw new SlackUnsupportedTagException(node.Name);
+            }
+            
             return $"{Environment.NewLine}* * *{Environment.NewLine}";
         }
     }

src/ReverseMarkdown/Converters/Img.cs

@@ -11,6 +11,11 @@ public Img(Converter converter) : base(converter)
 
         public override string Convert(HtmlNode node)
         {
+            if (Converter.Config.SlackFlavored)
+            {
+                throw new SlackUnsupportedTagException(node.Name);
+            }
+            
             var alt = node.GetAttributeValue("alt", string.Empty);
             var src = node.GetAttributeValue("src", string.Empty);
 

src/ReverseMarkdown/Converters/S.cs

@@ -20,7 +20,8 @@ public override string Convert(HtmlNode node)
                 return content;
             }
 
-            return content.EmphasizeContentWhitespaceGuard("~~");
+            var emphasis = Converter.Config.SlackFlavored ? "~" : "~~";
+            return content.EmphasizeContentWhitespaceGuard(emphasis);
         }
 
         private static bool AlreadyStrikethrough(HtmlNode node)

src/ReverseMarkdown/Converters/Strong.cs

@@ -27,7 +27,8 @@ public override string Convert(HtmlNode node)
                 ? " "
                 : "";
 
-            return content.EmphasizeContentWhitespaceGuard("**", spaceSuffix);
+            var emphasis = Converter.Config.SlackFlavored ? "*" : "**";
+            return content.EmphasizeContentWhitespaceGuard(emphasis, spaceSuffix);
         }
 
         private static bool AlreadyBold(HtmlNode node)

src/ReverseMarkdown/Converters/Sup.cs

@@ -12,6 +12,11 @@ public Sup(Converter converter) : base(converter)
 
         public override string Convert(HtmlNode node)
         {
+            if (Converter.Config.SlackFlavored)
+            {
+                throw new SlackUnsupportedTagException(node.Name);
+            }
+            
             var content = TreatChildren(node);
             if (string.IsNullOrEmpty(content) || AlreadySup(node))
             {

src/ReverseMarkdown/Converters/Table.cs

@@ -14,6 +14,11 @@ public Table(Converter converter) : base(converter)
 
         public override string Convert(HtmlNode node)
         {
+            if (Converter.Config.SlackFlavored)
+            {
+                throw new SlackUnsupportedTagException(node.Name);
+            }
+            
             // if table does not have a header row , add empty header row if set in config
             var useEmptyRowForHeader = this.Converter.Config.TableWithoutHeaderRowHandling ==
                                        Config.TableWithoutHeaderRowHandlingOption.EmptyRow;