[naaz234]

Description

HashHeaderProcessor.RE grows the header text one character at a time and after each step tries #*(?:\n|$) to see if only closing hashes are left. When that step lands inside a run of # that is followed by more text, the check scans to the end of the run and fails, so a run of n hashes costs O(n^2). With no extensions loaded, '# ' + '#' * 8000 + 'x' takes about 13 seconds in markdown.markdown() on CPython 3.11, and profiling puts all of it in re.search for this pattern. '#' * n + '\\' goes through the same path (over 9 s per search at n=8000).

This is not the same as #1545. There the time went to inline escape handling, while here it is spent inside the block regex.

The header now takes a whole run of hashes in one step when something other than a newline or the end of the block follows it (#+(?=[^#\n])), and # is removed from the single-character branch, so the lazy match can only stop at the start of a hash run. I compared the old and new patterns on every string of up to 8 characters over #, \, a and \n, plus about 470k random longer strings, and the span and groups matched in every case. The 8000-hash input now converts in about 1 ms and 100k hashes in about 0.3 s. The new test takes about 70 s against the old pattern and is instant with the fix. The full test suite and flake8 pass.

AI Assistance Disclosure

• No AI tools were used in preparing this PR.

• AI tools were used and I have disclosed which ones, and have fully reviewed and verified their output.

  AI tools were use to assist with the following:

  • Code
  • Code Comments
  • Documentation
  • Changelog Entry
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  • The Description Above
  • Other: finding the issue, the benchmarks and the equivalence check between the old and new regex

Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
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• I have added or updated relevant docs, including release notes if applicable which follow the Documentation Style Guide.
• I have added or updated relevant tests.
• I have not requested, and will not request, an automated AI review for this PR.