[parsing] Require whitespace in front of mfm italic nodes using asterisk or underscore delimiters

This ensures matching behavior with mfm.js

Iceshrimp.Parsing/Mfm.fs

@@ -207,12 +207,26 @@ module private MfmParser =
 
     // Node parsers
 
+    let italicNode1 =
+        previousCharSatisfiesNot isNotWhitespace
+        >>. italicPattern
+        >>. pushLine
+        >>. manyTill inlineNode italicPattern
+        .>> assertLine
+
+    let italicNode2 =
+        previousCharSatisfiesNot isNotWhitespace
+        >>. italicPatternAlt
+        >>. pushLine
+        >>. manyTill inlineNode italicPatternAlt
+        .>> assertLine
+
+    let italicNode3 =
+        skipString "<i>" >>. pushLine >>. manyTill inlineNode (skipString "</i>")
+        .>> assertLine
+
     let italicNode =
-        (italicPattern >>. pushLine >>. manyTill inlineNode italicPattern .>> assertLine)
+        italicNode1 <|> italicNode2 <|> italicNode3
-        <|> (italicPatternAlt >>. pushLine >>. manyTill inlineNode italicPatternAlt
-             .>> assertLine)
-        <|> (skipString "<i>" >>. pushLine >>. manyTill inlineNode (skipString "</i>")
-             .>> assertLine)
         |>> fun c -> MfmItalicNode(aggregateTextInline c) :> MfmNode
 
     let boldNode =
@@ -381,10 +395,7 @@ module private MfmParser =
           fnNode
           charNode ]
 
-    let simpleNodeSeq =
+    let simpleNodeSeq = [ plainNode; emojiCodeNode; charNode ]
-        [ plainNode
-          emojiCodeNode
-          charNode ]
 
     let blockNodeSeq =
         [ plainNode; centerNode; smallNode; codeBlockNode; mathBlockNode; quoteNode ]
@@ -395,12 +406,12 @@ module private MfmParser =
     do nodeRef.Value <- choice <| seqAttempt (seqFlatten <| nodeSeq)
 
     do inlineNodeRef.Value <- choice <| (seqAttempt inlineNodeSeq) |>> fun v -> v :?> MfmInlineNode
-    
+
     do simpleRef.Value <- choice <| seqAttempt simpleNodeSeq
 
     // Final parse command
     let parse = spaces >>. manyTill node eof .>> spaces
-    
+
     let parseSimple = spaces >>. manyTill simple eof .>> spaces
 
 open MfmParser

Iceshrimp.Tests/Parsing/MfmTests.cs

@@ -40,6 +40,32 @@ public class MfmTests
 		resMixedAlt.Should().Equal(expected, MfmNodeEqual);
 	}
 
+	[TestMethod]
+	public void TestItalicNegative()
+	{
+		List<MfmNode> expected    = [new MfmTextNode("test*test*test")];
+		List<MfmNode> expectedAlt = [new MfmTextNode("test_test_test")];
+
+		Mfm.parse("test*test*test").ToList().Should().Equal(expected, MfmNodeEqual);
+		Mfm.parse("test_test_test").ToList().Should().Equal(expectedAlt, MfmNodeEqual);
+
+		expected    = [new MfmTextNode("test*test* test")];
+		expectedAlt = [new MfmTextNode("test_test_ test")];
+
+		Mfm.parse("test*test* test").ToList().Should().Equal(expected, MfmNodeEqual);
+		Mfm.parse("test_test_ test").ToList().Should().Equal(expectedAlt, MfmNodeEqual);
+
+		expected =
+		[
+			new MfmTextNode("test "),
+			new MfmItalicNode(ListModule.OfSeq<MfmInlineNode>([new MfmTextNode("test")])),
+			new MfmTextNode("test")
+		];
+
+		Mfm.parse("test *test*test").ToList().Should().Equal(expected, MfmNodeEqual);
+		Mfm.parse("test _test_test").ToList().Should().Equal(expected, MfmNodeEqual);
+	}
+
 	[TestMethod]
 	public void TestParseList()
 	{