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gh-122055: Clarify documentation for empty matches in RE #133169

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16 changes: 9 additions & 7 deletions Doc/library/re.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -985,8 +985,8 @@ Functions
That way, separator components are always found at the same relative
indices within the result list.

Empty matches for the pattern split the string only when not adjacent
to a previous empty match.
Adjacent empty matches are not possible, but an empty match can occur
immediately after a non-empty match.

.. code:: pycon

Expand Down Expand Up @@ -1089,9 +1089,12 @@ Functions

The optional argument *count* is the maximum number of pattern occurrences to be
replaced; *count* must be a non-negative integer. If omitted or zero, all
occurrences will be replaced. Empty matches for the pattern are replaced only
when not adjacent to a previous empty match, so ``sub('x*', '-', 'abxd')`` returns
``'-a-b--d-'``.
occurrences will be replaced.

Adjacent empty matches are not possible, but an empty match can occur
immediately after a non-empty match.
As a result, ``sub('x*', '-', 'abxd')`` returns ``'-a-b--d-'``
instead of ``'-a-b-d-'``.

.. index:: single: \g; in regular expressions

Expand Down Expand Up @@ -1122,8 +1125,7 @@ Functions
.. versionchanged:: 3.7
Unknown escapes in *repl* consisting of ``'\'`` and an ASCII letter
now are errors.
Empty matches for the pattern are replaced when adjacent to a previous
non-empty match.
An empty match can occur immediately after a non-empty match.

.. versionchanged:: 3.12
Group *id* can only contain ASCII digits.
Expand Down
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