Fix an edgecase where substitutions could not track the delimiter pro…

…perly when an empty line preceded it and followed it (#155)

src/Elastic.Markdown/Myst/Substitution/SubstitutionParser.cs

@@ -135,6 +135,7 @@ public override bool Match(InlineProcessor processor, ref StringSlice slice)
 
 		// We've already skipped the opening sticks. Account for that here.
 		startPosition -= openSticks;
+		startPosition = Math.Max(startPosition, 0);
 
 		var key = content.ToString().Trim(['{', '}']);
 		var found = false;
@@ -145,11 +146,14 @@ public override bool Match(InlineProcessor processor, ref StringSlice slice)
 			replacement = value.ToString() ?? string.Empty;
 		}
 
+		var start = processor.GetSourcePosition(startPosition, out var line, out var column);
+		var end = processor.GetSourcePosition(slice.Start);
+		var sourceSpan = new SourceSpan(start, end);
+
 		var substitutionLeaf = new SubstitutionLeaf(content.ToString(), found, replacement)
 		{
-			Delimiter = slice.Text[startPosition],
-			Span = new SourceSpan(processor.GetSourcePosition(startPosition, out var line, out var column),
-				processor.GetSourcePosition(slice.Start)),
+			Delimiter = '{',
+			Span = sourceSpan,
 			Line = line,
 			Column = column,
 			DelimiterCount = openSticks

tests/Elastic.Markdown.Tests/Inline/SubstitutionTest.cs

@@ -133,3 +133,25 @@ public void OnlySeesGlobalVariable() =>
 	public void HasError() => Collector.Diagnostics.Should().HaveCount(1)
 		.And.Contain(d => d.Message.Contains("{hello-world} can not be redeclared in front matter as its a global substitution"));
 }
+
+public class ReplaceInHeader(ITestOutputHelper output) : InlineTest(output,
+"""
+---
+sub:
+  hello-world: "Hello World!"
+---
+
+## {{hello-world}} [#custom-anchor]
+
+"""
+)
+{
+
+	[Fact]
+	public void OnlySeesGlobalVariable() =>
+		Html.Should().Contain("<h2>Hello World!");
+
+	[Fact]
+	public void HasNoErrors() => Collector.Diagnostics.Should().HaveCount(0);
+
+}