fix: no strikethrough on different number of tildes (#1821)

Possible Breaking Change: When using the inline.del rule the text is provided in capture group 2 instead of 1.

src/Tokenizer.js

@@ -585,7 +585,7 @@ module.exports = class Tokenizer {
       return {
         type: 'del',
         raw: cap[0],
-        text: cap[1]
+        text: cap[2]
       };
     }
   }

src/rules.js

@@ -318,8 +318,8 @@ inline.gfm = merge({}, inline.normal, {
   _extended_email: /[A-Za-z0-9._+-]+(@)[a-zA-Z0-9-_]+(?:\.[a-zA-Z0-9-_]*[a-zA-Z0-9])+(?![-_])/,
   url: /^((?:ftp|https?):\/\/|www\.)(?:[a-zA-Z0-9\-]+\.?)+[^\s<]*|^email/,
   _backpedal: /(?:[^?!.,:;*_~()&]+|\([^)]*\)|&(?![a-zA-Z0-9]+;$)|[?!.,:;*_~)]+(?!$))+/,
-  del: /^~+(?=\S)([\s\S]*?\S)~+/,
+  del: /^(~~?)(?=[^\s~])([\s\S]*?[^\s~])\1(?=[^~]|$)/,
-  text: /^(`+|[^`])(?:(?= {2,}\n)|[\s\S]*?(?:(?=[\\<!\[`*~]|\b_|https?:\/\/|ftp:\/\/|www\.|$)|[^ ](?= {2,}\n)|[^a-zA-Z0-9.!#$%&'*+\/=?_`{\|}~-](?=[a-zA-Z0-9.!#$%&'*+\/=?_`{\|}~-]+@))|(?=[a-zA-Z0-9.!#$%&'*+\/=?_`{\|}~-]+@))/
+  text: /^([`~]+|[^`~])(?:(?= {2,}\n)|[\s\S]*?(?:(?=[\\<!\[`*~]|\b_|https?:\/\/|ftp:\/\/|www\.|$)|[^ ](?= {2,}\n)|[^a-zA-Z0-9.!#$%&'*+\/=?_`{\|}~-](?=[a-zA-Z0-9.!#$%&'*+\/=?_`{\|}~-]+@))|(?=[a-zA-Z0-9.!#$%&'*+\/=?_`{\|}~-]+@))/
 });
 
 inline.gfm.url = edit(inline.gfm.url, 'i')

test/specs/new/del_strikethrough.html

@@ -0,0 +1,16 @@
+<p><del>test</del></p>
+
+<p>~~test~</p>
+
+<p>~test~~</p>
+
+<p><del>test</del></p>
+
+<p><del>test
+test</del></p>
+
+<p>~~test</p>
+
+<p>test~~</p>
+
+<pre><code class="language-test~~~"></code></pre>

test/specs/new/del_strikethrough.md

@@ -0,0 +1,16 @@
+~~test~~
+
+~~test~
+
+~test~~
+
+~test~
+
+~~test
+test~~
+
+~~test
+
+test~~
+
+~~~test~~~