fix: resolve even-numbered backtick precedence issue (#3776) #3786

sanki92 · 2025-10-05T06:03:01Z

Marked version: 16.3.0

Markdown flavor: CommonMark

Description

Fixes Even-numbered backtick strings have incorrect precedence with at least emStrong and link delimiters #3776

Even-numbered backtick strings (2, 4, 6, etc.) weren't creating proper codespans, allowing emphasis to be incorrectly processed.

Before: **text ``**`` more** → text ```` more** ❌
After: **text ``**`` more** → **text <code>**</code> more** ✅

Root Cause

blockSkip regex only matched single backticks properly
Emphasis was processed before codespans (violates CommonMark precedence)

Solution

Updated blockSkip regex to handle multiple backticks (same pattern as inlineCode)
Changed tokenization order: codespans before emphasis
Added comprehensive unit tests

Changes Made

// src/rules.ts - Updated blockSkip regex
(`+)([^`]|[^`][\s\S]*?[^`])\1(?!`)

// src/Lexer.ts - Fixed tokenization order  
if (token = this.tokenizer.codespan(src)) { /* process first */ }
if (token = this.tokenizer.emStrong(src, maskedSrc, prevChar)) { /* process after */ }

Contributor

Test(s) exist to ensure functionality and minimize regression - Added test/unit/issue-3776-backtick-precedence.test.js with 6 comprehensive test cases covering:
- Double backticks (main issue case)
- Quadruple backticks
- Single & triple backticks (regression tests)
- Mixed emphasis and codespan scenarios
- Nested cases
Full test suite passes (1705/1705 tests)

Committer

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

- Update blockSkip regex to properly handle multiple backticks using inlineCode pattern - Change tokenization order to process codespans before emphasis (CommonMark precedence) - Add comprehensive unit tests covering main issue and edge cases - Fix ensures even-numbered backticks (2, 4, 6, etc.) create proper codespans Fixes markedjs#3776

vercel · 2025-10-05T06:03:07Z

@sanki92 is attempting to deploy a commit to the MarkedJS Team on Vercel.

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gemini-code-assist · 2025-10-05T06:03:14Z

Summary of Changes

Hello @sanki92, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical parsing issue in the Markdown renderer where codespans created with even-numbered backticks were not correctly recognized, allowing unintended emphasis formatting. The changes ensure strict adherence to CommonMark specifications by reordering token processing to prioritize codespans and enhancing the regular expression used for block skipping, thereby preventing incorrect rendering of inline code.

Highlights

Fixes Backtick Precedence Issue: Resolves a bug where even-numbered backticks (e.g., text) were not correctly creating codespans, leading to emphasis being incorrectly processed within them.
Root Cause Identified: The problem stemmed from the blockSkip regex not properly matching multiple backticks and emphasis being processed before codespans, violating CommonMark precedence rules.
Solution Implemented: The blockSkip regex has been updated to handle multiple backticks, and the tokenization order in Lexer.ts has been changed to prioritize codespans over emphasis.
Comprehensive Testing: A new unit test file (issue-3776-backtick-precedence.test.js) has been added with 6 test cases covering various scenarios, including double, quadruple, single, triple, mixed, and nested backtick/emphasis combinations.

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Code Review

This pull request correctly fixes a precedence issue where even-numbered backtick sequences were not being parsed as codespans correctly. The approach of reordering the tokenization logic and updating the blockSkip regex is sound, and the new unit tests are thorough. I've included a couple of suggestions to improve maintainability by reducing code duplication in both the regular expression definitions and the new tests.

gemini-code-assist · 2025-10-05T06:04:20Z

src/rules.ts


 // sequences em should skip over [title](link), `code`, <html>
-const blockSkip = /\[[^\[\]]*?\]\((?:\\[\s\S]|[^\\\(\)]|\((?:\\[\s\S]|[^\\\(\)])*\))*\)|`[^`]*?`|<(?! )[^<>]*?>/g;
+const blockSkip = /\[[^\[\]]*?\]\((?:\\[\s\S]|[^\\\(\)]|\((?:\\[\s\S]|[^\\\(\)])*\))*\)|(`+)([^`]|[^`][\s\S]*?[^`])\1(?!`)|<(?! )[^<>]*?>/g;


The regex pattern for matching code spans, (+)([^`]|[^`][\s\S]*?[^])\1(?!), is now duplicated from the inlineCode` rule (line 253). To improve maintainability and adhere to the DRY (Don't Repeat Yourself) principle, consider extracting this pattern into a shared constant. This would make future updates to the pattern easier and less error-prone.

For example, you could define a constant and reuse it:

const codeSpanPattern = '(`+)([^`]|[^`][\s\S]*?[^`])\1(?!`)'; const inlineCode = new RegExp(`^${codeSpanPattern}`); // ... const blockSkip = new RegExp(`\[[^\[\]]*?\]$(?:...)*$|${codeSpanPattern}|<(?! )[^<>]*?>`, 'g');

Great catch! 👍 Extracted the shared pattern into _codeSpanPattern to avoid duplication. Much cleaner now.

gemini-code-assist · 2025-10-05T06:04:20Z

test/unit/issue-3776-backtick-precedence.test.js

+  it('should prioritize codespans over emphasis for double backticks', () => {
+    const input = '**You might think this should be bold, but it should actually be regular text because codespans have higher priority: ``**``';
+    const html = marked(input);
+
+    // Should create a codespan containing **, not emphasis
+    assert.ok(html.includes('<code>**</code>'), 'Should contain codespan with **');
+    assert.ok(!html.includes('<strong>'), 'Should not contain strong tags');
+  });
+
+  it('should prioritize codespans over emphasis for quadruple backticks', () => {
+    const input = '**You might think this should be bold, but: ````**````';
+    const html = marked(input);
+
+    // Should create a codespan containing **, not emphasis
+    assert.ok(html.includes('<code>**</code>'), 'Should contain codespan with **');
+    assert.ok(!html.includes('<strong>'), 'Should not contain strong tags');
+  });
+
+  it('should continue working correctly for single backticks', () => {
+    const input = '**You might think this should be bold, but: `**`';
+    const html = marked(input);
+
+    // Should create a codespan containing **, not emphasis
+    assert.ok(html.includes('<code>**</code>'), 'Should contain codespan with **');
+    assert.ok(!html.includes('<strong>'), 'Should not contain strong tags');
+  });
+
+  it('should continue working correctly for triple backticks', () => {
+    const input = '**You might think this should be bold, but: ```**```';
+    const html = marked(input);
+
+    // Should create a codespan containing **, not emphasis
+    assert.ok(html.includes('<code>**</code>'), 'Should contain codespan with **');
+    assert.ok(!html.includes('<strong>'), 'Should not contain strong tags');
+  });


These four test cases are very similar, differing only in the number of backticks used. To make the test suite more concise and maintainable, you could refactor them into a single, data-driven test. This would reduce code duplication and make it easier to add more variations in the future.

Suggested change

it('should prioritize codespans over emphasis for double backticks', () => {

const input = '**You might think this should be bold, but it should actually be regular text because codespans have higher priority: ``**``';

const html = marked(input);

// Should create a codespan containing **, not emphasis

assert.ok(html.includes('<code>**</code>'), 'Should contain codespan with **');

assert.ok(!html.includes(''), 'Should not contain strong tags');

});

it('should prioritize codespans over emphasis for quadruple backticks', () => {

const input = '**You might think this should be bold, but: ````**````';

const html = marked(input);

// Should create a codespan containing **, not emphasis

assert.ok(html.includes('<code>**</code>'), 'Should contain codespan with **');

assert.ok(!html.includes(''), 'Should not contain strong tags');

});

it('should continue working correctly for single backticks', () => {

const input = '**You might think this should be bold, but: `**`';

const html = marked(input);

// Should create a codespan containing **, not emphasis

assert.ok(html.includes('<code>**</code>'), 'Should contain codespan with **');

assert.ok(!html.includes(''), 'Should not contain strong tags');

});

it('should continue working correctly for triple backticks', () => {

const input = '**You might think this should be bold, but: ```**```';

const html = marked(input);

// Should create a codespan containing **, not emphasis

assert.ok(html.includes('<code>**</code>'), 'Should contain codespan with **');

assert.ok(!html.includes(''), 'Should not contain strong tags');

});

const backtickTests = [

{ name: 'single', ticks: '`' },

{ name: 'double', ticks: '``' },

{ name: 'triple', ticks: '```' },

{ name: 'quadruple', ticks: '````' },

];

for (const { name, ticks } of backtickTests) {

it(`should prioritize codespans over emphasis for ${name} backticks`, () => {

const input = `**You might think this should be bold, but: ${ticks}**${ticks}`;

const html = marked(input);

// Should create a codespan containing **, not emphasis

assert.ok(html.includes('<code>**</code>'), 'Should contain codespan with **');

assert.ok(!html.includes(''), 'Should not contain strong tags');

});

}

Perfect suggestion! Refactored into a data-driven test with a loop. Way more maintainable - thanks!

- Extract shared codespan pattern to reduce duplication (DRY principle) - Refactor tests into data-driven format for maintainability - Improve code consistency between inlineCode and blockSkip regex

UziTech

use test/specs/new for markdown tests
don't rearrange Lexer
use the edit method instead of new Regexp

…ve tests to specs/new

sanki92 · 2025-10-05T06:47:36Z

Hi @UziTech,

Thanks for the feedback! I've addressed all your points:

Updated blockSkip to use edit().replace('codePattern', inlineCode).getRegex() instead of new RegExp
Reverted to original tokenization order (emStrong before codespan)
Created proper markdown spec tests in test/specs/new/issue_3776_backtick_precedence.{md,html} and removed the unit test

The fix still works correctly. All 1707 tests are passing.

UziTech

This regexp has a redos vulnerability

You can check by pasting the regexp into https://makenowjust-labs.github.io/recheck/playground/

the full regexp:

/\[[^\[\]]*?\]\((?:\\[\s\S]|[^\\\(\)]|\((?:\\[\s\S]|[^\\\(\)])*\))*\)|(`+)([^`]|[^`][\s\S]*?[^`])\1(?!`)|<(?! )[^<>]*?>/

- Replace complex codespan pattern with simple ReDoS-safe version - Change from vulnerable (+)([^]|[^][\s\S]*?[^])\1(?!) - To safe pattern \+[\s\S]*?\+ without nested quantifiers

sanki92 · 2025-10-07T06:21:25Z

Hi @UziTech,

Thanks for catching the ReDoS vulnerability! I've fixed it by replacing the complex codespan pattern with a much simpler, ReDoS-safe version.

The new pattern:

Eliminates nested quantifiers and problematic alternation
Maintains all original functionality
All 1707 tests continue to pass

sanki92 · 2025-10-11T06:34:41Z

Hi @UziTech, just following up on this PR. I’ve addressed the ReDoS vulnerability and all tests are passing. If everything looks good to you and there’s nothing else to change, could you add the hacktoberfest-accepted label so it counts for the event? Thanks!

vercel · 2025-10-12T04:44:10Z

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Preview	Comments	Updated (UTC)
marked-website	Ready	Preview	Comment	Oct 15, 2025 6:03am

UziTech · 2025-10-12T04:57:11Z

This actually breaks some markdown like

**This should be bold ``**`

This PR

latest marked

commonmark

- Change alternation order from shortest to longest to ensure proper matching - Use pattern: ``[^]*?``|`[^]*?`|`[^]*?`|[^]*? - Fixes case where **This should be bold `** was incorrectly blocked - Maintains fix for original issue: **text `**` correctly creates codespan - All 1707 tests pass, ReDoS-safe pattern verified

sanki92 · 2025-10-12T06:11:35Z

Fixed ✅

UziTech · 2025-10-14T06:04:44Z

I think the codepattern regexp should be (?<!`)(`+)[^`]+\1(?!`) this is safe and will ensure the same amount of backticks in the front and back.

Looks like you need to rebase to resolve conflicts with the latest version

Can you also add the following to the test:

<p><strong>This should be bold ``</strong>`</p>
<p><strong>This should be bold `</strong>``</p>

**This should be bold ``**`

**This should be bold `**``

sanki92 · 2025-10-15T03:07:04Z

Done ✅

src/rules.ts

Co-authored-by: Tony Brix <[email protected]>

UziTech

Thanks 💯

gemini-code-assist bot reviewed Oct 5, 2025

View reviewed changes

refactor: address gemini-code-assist review feedback

7a38751

- Extract shared codespan pattern to reduce duplication (DRY principle) - Refactor tests into data-driven format for maintainability - Improve code consistency between inlineCode and blockSkip regex

UziTech requested changes Oct 5, 2025

View reviewed changes

Address UziTech's feedback: use edit method, preserve Lexer order, mo…

0dacbf4

…ve tests to specs/new

UziTech requested changes Oct 7, 2025

View reviewed changes

fix: resolve ReDoS vulnerability in blockSkip regex pattern

69a1966

- Replace complex codespan pattern with simple ReDoS-safe version - Change from vulnerable (+)([^]|[^][\s\S]*?[^])\1(?!) - To safe pattern \+[\s\S]*?\+ without nested quantifiers

vercel bot deployed to Preview October 12, 2025 04:44 View deployment

fix: update regex pattern and add test case

1c8089b

sanki92 force-pushed the fix-issue-3776-backtick-precedence branch from b57392c to 1c8089b Compare October 14, 2025 14:33

Merge branch 'master' into fix-issue-3776-backtick-precedence

5348c18

vercel bot deployed to Preview October 15, 2025 04:57 View deployment

UziTech requested changes Oct 15, 2025

View reviewed changes

src/rules.ts Outdated Show resolved Hide resolved

fix: update blockSkip regex

6b5144c

Co-authored-by: Tony Brix <[email protected]>

vercel bot deployed to Preview October 15, 2025 06:03 View deployment

UziTech approved these changes Oct 15, 2025

View reviewed changes

UziTech added the hacktoberfest-accepted label Oct 15, 2025

UziTech requested review from calculuschild and styfle October 15, 2025 06:06

fix: resolve even-numbered backtick precedence issue (#3776) #3786

fix: resolve even-numbered backtick precedence issue (#3776) #3786

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