Add bracket handling to Markdown link format

Allow users to put both balanced brackets, as well as unbalanced
escaped ones, in Markdown link text. The implementation is the exact
same as in the case of new format links.

For completion's sake, tests also included.

osu.Game.Tests/Chat/MessageFormatterTests.cs

@@ -179,6 +179,53 @@ public void TestMarkdownFormatLink()
             Assert.AreEqual(11, result.Links[0].Length);
         }
 
+        [Test]
+        public void TestMarkdownFormatLinkWithBalancedBrackets()
+        {
+            Message result = MessageFormatter.FormatMessage(new Message { Content = "This is a [tricky [one]](https://osu.ppy.sh)!" });
+
+            Assert.AreEqual("This is a tricky [one]!", result.DisplayContent);
+            Assert.AreEqual(1, result.Links.Count);
+            Assert.AreEqual("https://osu.ppy.sh", result.Links[0].Url);
+            Assert.AreEqual(10, result.Links[0].Index);
+            Assert.AreEqual(12, result.Links[0].Length);
+        }
+
+        [Test]
+        public void TestMarkdownFormatLinkWithEscapedBrackets()
+        {
+            Message result = MessageFormatter.FormatMessage(new Message { Content = "This is [another loose bracket \\]](https://osu.ppy.sh)." });
+
+            Assert.AreEqual("This is another loose bracket ].", result.DisplayContent);
+            Assert.AreEqual(1, result.Links.Count);
+            Assert.AreEqual("https://osu.ppy.sh", result.Links[0].Url);
+            Assert.AreEqual(8, result.Links[0].Index);
+            Assert.AreEqual(23, result.Links[0].Length);
+        }
+
+        [Test]
+        public void TestMarkdownFormatWithBackslashes()
+        {
+            Message result = MessageFormatter.FormatMessage(new Message { Content = "This link [should end with a backslash \\](https://osu.ppy.sh)." });
+            Assert.AreEqual("This link should end with a backslash \\.", result.DisplayContent);
+            Assert.AreEqual(1, result.Links.Count);
+            Assert.AreEqual("https://osu.ppy.sh", result.Links[0].Url);
+            Assert.AreEqual(10, result.Links[0].Index);
+            Assert.AreEqual(29, result.Links[0].Length);
+        }
+
+        [Test]
+        public void TestMarkdownFormatLinkWithEscapedAndBalancedBrackets()
+        {
+            Message result = MessageFormatter.FormatMessage(new Message { Content = "This is a [\\]super\\[\\[ tricky [one]](https://osu.ppy.sh)!" });
+
+            Assert.AreEqual("This is a ]super[[ tricky [one]!", result.DisplayContent);
+            Assert.AreEqual(1, result.Links.Count);
+            Assert.AreEqual("https://osu.ppy.sh", result.Links[0].Url);
+            Assert.AreEqual(10, result.Links[0].Index);
+            Assert.AreEqual(21, result.Links[0].Length);
+        }
+
         [Test]
         public void TestChannelLink()
         {

osu.Game.Tests/Visual/Online/TestSceneChatLink.cs

@@ -128,6 +128,7 @@ private void testLinksGeneral()
             addMessageWithChecks("Let's (try)[https://osu.ppy.sh/home] [https://osu.ppy.sh/b/252238 multiple links] https://osu.ppy.sh/home", 3,
                 expectedActions: new[] { LinkAction.External, LinkAction.OpenBeatmap, LinkAction.External });
             addMessageWithChecks("[https://osu.ppy.sh/home New link format with escaped [and \\[ paired] braces]", expectedActions: LinkAction.External);
+            addMessageWithChecks("[Markdown link format with escaped [and \\[ paired] braces](https://osu.ppy.sh/home)", expectedActions: LinkAction.External);
             // note that there's 0 links here (they get removed if a channel is not found)
             addMessageWithChecks("#lobby or #osu would be blue (and work) in the ChatDisplay test (when a proper ChatOverlay is present).");
             addMessageWithChecks("I am important!", 0, false, true);

osu.Game/Online/Chat/MessageFormatter.cs

@@ -20,7 +20,7 @@ public static class MessageFormatter
         private static readonly Regex new_link_regex = new Regex(@"\[(?<url>[a-z]+://[^ ]+) (?<text>(((?<=\\)[\[\]])|[^\[\]])*(((?<open>\[)(((?<=\\)[\[\]])|[^\[\]])*)+((?<close-open>\])(((?<=\\)[\[\]])|[^\[\]])*)+)*(?(open)(?!)))\]");
 
         // [test](https://osu.ppy.sh/b/1234) -> test (https://osu.ppy.sh/b/1234) aka correct markdown format
-        private static readonly Regex markdown_link_regex = new Regex(@"\[(?<text>[^\]]*)\]\((?<url>[a-z]+://[^ ]+)\)");
+        private static readonly Regex markdown_link_regex = new Regex(@"\[(?<text>(((?<=\\)[\[\]])|[^\[\]])*(((?<open>\[)(((?<=\\)[\[\]])|[^\[\]])*)+((?<close-open>\])(((?<=\\)[\[\]])|[^\[\]])*)+)*(?(open)(?!)))\]\((?<url>[a-z]+://[^ ]+)\)");
 
         // advanced, RFC-compatible regular expression that matches any possible URL, *but* allows certain invalid characters that are widely used
         // This is in the format (<required>, [optional]):
@@ -191,7 +191,7 @@ private static MessageFormatterResult format(string toFormat, int startIndex = 0
             handleMatches(new_link_regex, "{1}", "{2}", result, startIndex, escapeChars: new[] { '[', ']' });
 
             // handle the standard markdown []() format
-            handleMatches(markdown_link_regex, "{1}", "{2}", result, startIndex);
+            handleMatches(markdown_link_regex, "{1}", "{2}", result, startIndex, escapeChars: new[] { '[', ']' });
 
             // handle the ()[] link format
             handleMatches(old_link_regex, "{1}", "{2}", result, startIndex);