add parsing for YAML frontmatter

This doesn't parse the YAML, but recognizes the YAML markers in
markdown.

__tests__/extensions.ts

@@ -21,6 +21,9 @@ const specParser = new SpecParser(parser, {
   fM: "FootnoteMark",
   fL: "FootnoteLabel",
   FR: "FootnoteReference",
+  YF: "YAMLFrontMatter",
+  ym: "YAMLMarker",
+  yc: "YAMLContent",
 });
 
 function test(name: string, spec: string, p = parser, only = false) {
@@ -162,4 +165,83 @@ Line 5}
 {BL:{LI:{l:-} {P:Line 6}}}
   `
   );
+
+  test(
+    "Frontmatter",
+    `
+{YF:{ym:---}
+{yc:tags: blah}
+{ym:---}}
+
+{HR:---}
+
+{P:some text}
+
+{SH2:A header
+{h:---}}
+  `
+  );
+
+  test(
+    "Frontmatter (trailing text)",
+    `
+{YF:{ym:---}
+{yc:tags: blah}
+{ym:---}}{P:test}
+
+{HR:---}
+
+{P:some text}
+
+{SH2:A header
+{h:---}}
+  `
+  );
+
+  test(
+    "Not Frontmatter (no close)",
+    `
+{HR:---}
+
+{P:some text}
+
+{SH1:A header
+{h:===}}
+  `
+  );
+
+  test(
+    "Not Frontmatter (close indented)",
+    `
+{HR:---}
+
+{P:some text}
+
+ {HR:---}
+  `
+  );
+
+  test(
+    "Not Frontmatter (space after open)",
+    `
+{HR:--- }
+
+{P:some text}
+
+{HR:---}
+  `
+  );
+
+  test(
+    "Not Frontmatter (data before open)",
+    `
+{P:some text}
+
+{HR:---}
+
+{P:some text}
+
+{HR:---}
+  `
+  );
 });

src/extensions.ts

@@ -1,3 +1,4 @@
+import { Input, PartialParse, Tree } from "@lezer/common";
 import {
   BlockContext,
   Element,
@@ -11,6 +12,13 @@ import {
   Table,
 } from "@lezer/markdown";
 
+declare module "@lezer/markdown" {
+  class BlockContext {
+    readonly input: Input;
+    checkedYaml: boolean | null;
+  }
+}
+
 /*
   Copyright (C) 2020 by Marijn Haverbeke <marijnh@gmail.com> and others
   https://github.com/lezer-parser/markdown/blob/f49eb8c8c82cfe45aa213ca1fe2cebc95305b88b/LICENSE
@@ -279,8 +287,42 @@ export const Footnote: MarkdownConfig = {
   ],
 };
 
+export const YAMLFrontMatter: MarkdownConfig = {
+  defineNodes: ["YAMLFrontMatter", "YAMLMarker", "YAMLContent"],
+  parseBlock: [
+    {
+      name: "YAMLFrontMatter",
+      parse(cx, line) {
+        if (cx.checkedYaml) {
+          return false;
+        }
+        cx.checkedYaml = true;
+        const fmRegex = /(^|^\s*\n)(---\n.+?\n---)/s;
+        const match = fmRegex.exec(cx.input.chunk(0));
+        if (match) {
+          const start = match[1].length;
+          const end = start + match[2].length;
+          cx.addElement(
+            cx.elt("YAMLFrontMatter", start, end, [
+              cx.elt("YAMLMarker", start, start + 3),
+              cx.elt("YAMLContent", start + 4, end - 4),
+              cx.elt("YAMLMarker", end - 3, end),
+            ])
+          );
+          while (cx.lineStart + line.text.length < end && cx.nextLine()) {}
+          line.pos = 3;
+          return true;
+        }
+        return false;
+      },
+      before: "LinkReference",
+    },
+  ],
+};
+
 export const ObsidianMDExtensions = [
   Footnote,
+  YAMLFrontMatter,
   InternalLink,
   Strikethrough,
   Table,