fix: fix strikethrough flanking rules

[UziTech]

Marked version: 17.0.1

Markdown flavor: GitHub Flavored Markdown

Description

Copy asterisk (*) emStrong flanking rules for tilde (~) strikethrough

• Fixes Tilde/strikethrough is not rendered the same way as GitHub #3863

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• no tests required for this PR.
• If submitting new feature, it has been documented in the appropriate places.

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[gemini-code-assist]

Summary of Changes

Hello @UziTech, 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 an issue where strikethrough parsing was overly permissive, leading to unintended rendering in specific text patterns. It overhauls the strikethrough tokenization process to strictly adhere to GitHub Flavored Markdown's flanking rules, ensuring that tilde characters are only interpreted as strikethrough delimiters when properly positioned and surrounded by appropriate characters. This change improves the accuracy and consistency of Markdown rendering, particularly for content that might contain tildes in non-strikethrough contexts.

Highlights

• Strikethrough Flanking Rules: The strikethrough (~) tokenization logic has been refactored to correctly apply GitHub Flavored Markdown (GFM) flanking rules, preventing incorrect rendering of tildes in contexts like numbers or mathematical expressions.
• Tokenizer Logic Update: The del method in the Tokenizer has been completely rewritten to use a more robust delimiter-matching algorithm, similar to how emphasis and strong text are handled, considering surrounding characters and delimiter lengths.
• New Regular Expressions: New GFM-specific regular expressions (_punctuationGfmDel, _punctuationOrSpaceGfmDel, _notPunctuationOrSpaceGfmDel, delLDelim, delRDelim) have been introduced to precisely define the left and right delimiters for strikethrough.
• Test Coverage: New test cases (del_flanking.md and del_flanking.html) have been added to validate the correct application of the strikethrough flanking rules and prevent regressions.

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[gemini-code-assist]

Code Review

This pull request implements GFM-style flanking rules for tilde-based strikethrough, mirroring the existing logic for asterisks in emphasis and strong tags. A security audit found no high or critical severity vulnerabilities, covering ReDoS, data flow, Unicode handling, and flanking rule logic. The overall approach is sound, but there are minor improvements needed in the new del tokenizer method in src/Tokenizer.ts, specifically regarding some dead code, a misleading comment, and a line that can be simplified for readability. After these minor changes, the PR should be in great shape.

[UziTech]

/gemini review

[gemini-code-assist]

Code Review

This pull request correctly implements the GFM flanking rules for strikethrough by adapting the existing logic from emphasis/strong. This is a great step towards better GFM compliance. However, this has been achieved by duplicating a significant amount of complex code and regular expressions. My review focuses on suggesting refactoring opportunities to remove this duplication, which will improve the long-term maintainability and robustness of the codebase.