re.sub does only first 16 replacements if re.S is used

[mgdelmonte]

BPO	22760
Nosy	@birkenfeld, @vstinner, @ezio-melotti, @serhiy-storchaka
Superseder	bpo-11957: re.sub confusion between count and flags args

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2014-10-29.15:51:11.671>
created_at = <Date 2014-10-29.15:35:07.080>
labels = ['expert-regex', 'type-bug']
title = 're.sub does only first 16 replacements if re.S is used'
updated_at = <Date 2014-10-29.16:09:48.499>
user = 'https://bugs.python.org/mgdelmonte'

bugs.python.org fields:

activity = <Date 2014-10-29.16:09:48.499>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2014-10-29.15:51:11.671>
closer = 'georg.brandl'
components = ['Regular Expressions']
creation = <Date 2014-10-29.15:35:07.080>
creator = 'mgdelmonte'
dependencies = []
files = []
hgrepos = []
issue_num = 22760
keywords = []
message_count = 5.0
messages = ['230216', '230217', '230218', '230219', '230220']
nosy_count = 7.0
nosy_names = ['georg.brandl', 'peter.otten', 'vstinner', 'ezio.melotti', 'mrabarnett', 'serhiy.storchaka', 'mgdelmonte']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = None
status = 'closed'
superseder = '11957'
type = 'behavior'
url = 'https://bugs.python.org/issue22760'
versions = ['Python 2.7']

[mgdelmonte]

Easily reproduced:

re.sub('x', 'a', "x"*20, re.S)

returns 'aaaaaaaaaaaaaaaaxxxx'

[peterotten]

This is not a bug; the signature of re.sub() is

sub(pattern, repl, string, count=0, flags=0)

and the fourth argument thus the 'count' delimiting the number of substitutions that you accidentally specify as

>>> import re
>>> re.S
16

I recommend that you use a keyword arg to fix your code:

>>> re.sub('x', 'a', "x"*20, flags=re.S)
'aaaaaaaaaaaaaaaaaaaa'

[birkenfeld]

The fourth parameter is not "flags", but "count", the max. number of substitutions. "flags" is the fifth parameter, or give it as a keyword argument.

[vstinner]

This bug report is common. An enhancement would be to make the count parameter a keyword only parameter. Would it break a lot of code?

[ezio-melotti]

See bpo-11957.