fix: only close a code fence with the opening fence character - #4164

Marked version: 18.1.1 (master at 4b7b5b2)

Markdown flavor: CommonMark

Description

No existing issue that I could find.

Expectation

A code block runs "until a closing code fence of the same type as the code block began with" (spec, 4.5), and a closing fence "may be followed only by spaces or tabs". So ```~~~ does not close a backtick fence, and the block continues:

```
code
```~~~
still code
```

<pre><code>code
```~~~
still code
</code></pre>

commonmark.js and micromark both give this.

Result

<pre><code>code
</code></pre>
<p>still code</p>
...

The closing part of block.fences was \1[~`]*, which accepts any mix of the two characters after the opening run.

Change

\1[~`]* → (?:(?=`)\1`*|(?=~)\1~*): a lookahead picks the opening character, so only more of it can follow. No lookbehind, so nothing changes for engines without it.

• npm run test:specs / test:unit / lint pass; new spec test/specs/new/fence_closing_mixed_characters.
• npm run bench (7 interleaved runs, median): master 2707 ms, this change within noise.
• Timing on long inputs stays linear, and the repeated ```~ case got faster (16k lines: 8 ms → 1 ms).

Found while comparing marked with commonmark.js and micromark on fuzzed input; written with help from an AI assistant (Claude) and checked against marked's test suite as described above.

Contributor

• Test(s) exist to ensure functionality and minimize regression
• no tests required for this PR.
• If submitting new feature, it has been documented in the appropriate places.

Committer

In most cases, this should be a different person than the contributor.

• CI is green (no forced merge required).
• Squash and Merge PR following conventional commit guidelines.