fix: markdown editor crashes on older iOS/Safari due to unsupported regex

[cemreinanc]

Fixes #4238

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Improved dollar sign escaping in the markdown editor to prevent double-escaping and better preserve escaped mathematical expressions.

• Tests

  • Added test coverage for edge cases in dollar sign and escaped delimiter handling.

[github-actions]

Cleanup: Preview Environment Removed

The preview environment for this PR has been destroyed.

Resource	Status
🌐 Preview App	Deleted
🗄️ PostgreSQL Branch	Deleted
⚡ Redis Database	Deleted
🔧 GitHub Deployments	Removed
📦 Docker Image	Retained (auto-cleanup via GHCR policies)

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Cleanup triggered by PR close at 2026-02-16T11:45:27Z

[ncarazon]

LGTM

[coderabbitai]

📝 Walkthrough

Walkthrough

This pull request fixes a critical compatibility issue affecting older iOS/Safari versions by removing unsupported negative lookbehind regex patterns from the markdown editor. The math expression preprocessing logic is restructured to use position-aware escaping detection instead of lookaround assertions, with corresponding updates to dollar sign escaping and new test coverage for edge cases.

Changes

Cohort / File(s)	Summary
Math Expression Preprocessing front_end/src/components/markdown_editor/helpers.ts	Replaced negative lookbehind regex patterns with simplified regexes and introduced isEscapedAt() helper for context-aware escaping detection. Restructured math placeholder management to directly inspect character positions instead of lookaround assertions, maintaining equivalent escaping behavior across browser versions.
Dollar Sign Escaping front_end/src/components/markdown_editor/plugins/equation/components/equation_editor.tsx	Replaced regex-based dollar sign escaping with callback-based iteration that checks for preceding backslashes, avoiding negative lookbehind syntax while preserving escaping logic.
Test Coverage front_end/src/components/markdown_editor/__tests__/helpers.test.ts	Added test cases validating double-escaping prevention and escaped block delimiter preservation in the dollar sign escaping suite.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Poem

🐰 Farewell, lookbehinds! A hop and a skip,
We've caught those pesky dollars mid-trip,
With escaping checks in every position,
Safari now dances with our ambition! 🌟

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the primary change: removing unsupported regex features (negative lookbehind) that cause crashes on older iOS/Safari versions.
Linked Issues check	✅ Passed	The PR successfully removes negative lookbehind syntax from BLOCK_MATH_REGEX and INLINE_MATH_REGEX, implementing alternative escaping logic with isEscapedAt helper to prevent crashes on Safari < 16.4 [#4238].
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the regex compatibility issue: regex simplification, escaping helper implementation, and corresponding test coverage for edge cases.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into main

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/4238-markdown-editor-regex

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[coderabbitai]

🧹 Nitpick comments (1)

front_end/src/components/markdown_editor/plugins/equation/components/equation_editor.tsx (1)

19-24: Good: removes negative lookbehind for Safari compatibility.

One minor note: this only checks a single preceding \, whereas helpers.ts uses isEscapedAt which correctly handles consecutive backslashes (e.g., \\$ where the backslash itself is escaped). In the equation editor context this is unlikely to matter, but importing and reusing isEscapedAt would improve consistency.

♻️ Optional: use isEscapedAt for consistency

+import { isEscapedAt } from "../../helpers"; // would need to export it
+
 value = value.replace(/\$/g, (match, offset, input) => {
-  if (offset > 0 && input[offset - 1] === "\\") {
+  if (isEscapedAt(input, offset)) {
     return match;
   }
   return "\\$";
 });