Fix crash on regexp special characters as markers

Author: wooorm

lib/index.js

@@ -13,6 +13,7 @@
 
 import {ok as assert} from 'devlop'
 import {toMatters} from 'micromark-extension-frontmatter'
+import escapeStringRegexp from 'escape-string-regexp'
 
 /**
  * Create an extension for `mdast-util-from-markdown`.
@@ -108,7 +109,7 @@ export function frontmatterToMarkdown(options) {
     unsafe.push({
       atBreak: true,
       character: open.charAt(0),
-      after: open.charAt(1)
+      after: escapeStringRegexp(open.charAt(1))
     })
   }
 

package.json

@@ -37,6 +37,7 @@
   "dependencies": {
     "@types/mdast": "^4.0.0",
     "devlop": "^1.0.0",
+    "escape-string-regexp": "^5.0.0",
     "mdast-util-from-markdown": "^2.0.0",
     "mdast-util-to-markdown": "^2.0.0",
     "micromark-extension-frontmatter": "^2.0.0"

test.js

@@ -340,6 +340,32 @@ test('frontmatterFromMarkdown', async function (t) {
       })
     }
   )
+
+  await t.test(
+    'should support regexp special characters as markers',
+    async function () {
+      const funky = {type: 'funky', marker: '*'}
+      const tree = fromMarkdown('***\na\n***\n\n*a', {
+        extensions: [frontmatter(funky)],
+        mdastExtensions: [frontmatterFromMarkdown(funky)]
+      })
+
+      removePosition(tree, {force: true})
+
+      assert.deepEqual(tree, {
+        type: 'root',
+        children: [
+          {type: 'funky', value: 'a'},
+          {type: 'paragraph', children: [{type: 'text', value: '*a'}]}
+        ]
+      })
+
+      assert.deepEqual(
+        toMarkdown(tree, {extensions: [frontmatterToMarkdown(funky)]}),
+        '***\na\n***\n\n\\*a\n'
+      )
+    }
+  )
 })
 
 test('frontmatterToMarkdown', async function (t) {