I see three problems:

1)The [^>] in (?=[^>]*\n) can match newlines.

2) ">" isn't allowed anywhere on the ">>>" line (after the ">>>"). Is that intentional?

3) The second negative lookahead is currently anchored to the character right after the ">>>", not to the start of the next line.

Fixes:

s/
   (?<!^>>[^>])
   ^>>>
   (?=
      [^\n]*\n      # <-- removed < and added first \n
      (?!^>>[^>])   # <-- moved into (?=...)
   )
/PYTHON_PROMPT/xmg;

[download]

Updated

In reply to Re: Using a regex to catch interpreters by ikegami
in thread Using a regex to catch interpreters by eastcoastcoder

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