[issue40027] re.sub inconsistency beginning with 3.7

[Brian]

Brian  added the comment:

txt = ' test'
txt = re.sub(r'^\s*', '^', txt)

substitutes once because the * is greedy.

txt = ' test'
txt = re.sub(r'^\s*?', '^', txt)

substitutes twice, consistent with the \Z behavior.

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[issue40027] re.sub inconsistency beginning with 3.7

[Brian]

Brian  added the comment:

I just ran into this change in behavior myself.

It's worth noting that the new behavior appears to match perl's behavior:

# perl -e 'print(("he" =~ s/e*\Z/ah/rg), "\n")'
hahah

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nosy: +bsammon

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[issue40027] re.sub inconsistency beginning with 3.7

[Wayne Davison]

Wayne Davison <4way...@gmail.com> added the comment:

Can this bug please be reopened and fixed? This is an anchored substitution, 
and so should never match more than once.

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nosy: +4wayned

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[issue40027] re.sub inconsistency beginning with 3.7

[Wayne Davison]

Wayne Davison  added the comment:

Another argument in favor of this being a bug, this does not exhibit the same 
doubling:

txt = ' test'
txt = re.sub(r'^\s*', '^', txt)

That always substitutes once.

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[issue40027] re.sub inconsistency beginning with 3.7

[Wayne Davison]

Wayne Davison  added the comment:

This is not the same thing because the match is anchored, so it is not adjacent 
to the prior match -- it is the same match. I think that r'\s*\Z' should behave 
the same way as r'\s*x' due to the anchor point. The current behavior is 
matching the same \Z twice.

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[issue40027] re.sub inconsistency beginning with 3.7

[Matthew Barnett]

Matthew Barnett  added the comment:

Duplicate of Issue39687.

See https://docs.python.org/3/library/re.html#re.sub and 
https://docs.python.org/3/whatsnew/3.7.html#changes-in-the-python-api.

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resolution:  -> duplicate
stage:  -> resolved
status: open -> closed

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[issue40027] re.sub inconsistency beginning with 3.7

[Wayne Davison]

New submission from Wayne Davison :

There is an inconsistency in re.sub() when substituting at the end of a string 
using a prior match with a '*' qualifier: the substitution now occurs twice.  
For example:

txt = re.sub(r'\s*\Z', "\n", txt)

This should work like txt.rstrip() + "\n", but beginning in 3.7, the re.sub 
version now matches twice and changes any non-empty whitespace into "\n\n" 
instead of "\n". (If there is no trailing whitespace it only matches once.)

The bug is the same if '$' is used instead of '\Z', but it does not happen if 
an actual character is specified (e.g. a substitution of r'\s*x' does not 
substitute twice if x has preceding whitespace).

I tested 2.7.17, 3.6.9, 3.7.7, 3.8.2, and 3.9.0a4, and it starts to fail in 
3.7.7 and beyond.

Attached is a test program.

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components: Regular Expressions
files: sub-bug.py
messages: 364688
nosy: Wayne Davison, ezio.melotti, mrabarnett
priority: normal
severity: normal
status: open
title: re.sub inconsistency beginning with 3.7
type: behavior
versions: Python 3.7, Python 3.8, Python 3.9
Added file: https://bugs.python.org/file48990/sub-bug.py

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