Fix collapsing of backslashes with other escapes

Closes GH-14.

Author: wooorm

lib/unsafe.js

@@ -82,7 +82,7 @@ module.exports = [
   },
   // A backslash can start an escape (when followed by punctuation) or a
   // hard break (when followed by an eol).
-  {character: '\\', after: '[!-/:-@[-`{-~]'},
+  {character: '\\', after: '[!-/:-@[-`{-~]', collapse: false},
   {character: '\\', after: '[\\r\\n]', inConstruct: 'phrasing'},
   // A right bracket can exit labels.
   {

lib/util/safe.js

@@ -30,8 +30,21 @@ function safe(context, input, config) {
     expression = compilePattern(pattern)
 
     while ((match = expression.exec(value))) {
-      before = 'before' in pattern || pattern.atBreak
-      after = 'after' in pattern
+      // Often, patterns which depend on a character before or after it, such
+      // as `!` when followed by `[` (for an image), do not have to be escaped
+      // when the other character has to be escaped as well.
+      // But in the case of a backslash, such as `\` when followed by `*`, that
+      // is not correct: both have to be escaped.
+      // This is a naïve “fix” for that though, but this seems the simplest for
+      // now.
+      if (pattern.collapse === false) {
+        before = false
+        after = false
+      } else {
+        before = 'before' in pattern || pattern.atBreak
+        after = 'after' in pattern
+      }
+
       position = match.index + (before ? match[1].length : 0)
 
       if (positions.indexOf(position) === -1) {

package.json

@@ -52,6 +52,7 @@
     "remark-preset-wooorm": "^8.0.0",
     "tape": "^5.0.0",
     "tinyify": "^3.0.0",
+    "unist-util-remove-position": "^3.0.0",
     "xo": "^0.37.0"
   },
   "scripts": {

test.js

@@ -1,4 +1,5 @@
 var test = require('tape')
+var remove = require('unist-util-remove-position')
 var from = require('mdast-util-from-markdown')
 var to = require('.')
 
@@ -2760,5 +2761,17 @@ test('roundtrip', function (t) {
   t.equal(to(from(step1)), step2, 'should roundtrip backslashes (1)')
   t.equal(to(from(step2)), step2, 'should roundtrip backslashes (2)')
 
+  doc = '\\\\\\*a\n'
+
+  t.equal(to(from(doc)), doc, 'should not collapse escapes (1)')
+
+  doc = '\\\\*a\\\\\\*'
+
+  t.deepEqual(
+    remove(from(doc)),
+    remove(from(to(from(doc)))),
+    'should not collapse escapes (2)'
+  )
+
   t.end()
 })