re.sub ignores flag re.M

[jlaurens]

BPO	42473

^{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 2020-11-26.16:01:39.446>
created_at = <Date 2020-11-26.13:47:10.045>
labels = ['invalid', 'type-bug', '3.8', 'expert-IO']
title = 're.sub ignores flag re.M'
updated_at = <Date 2020-11-26.16:01:39.442>
user = 'https://bugs.python.org/jlaurens'

bugs.python.org fields:

activity = <Date 2020-11-26.16:01:39.442>
actor = 'mrabarnett'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-26.16:01:39.446>
closer = 'mrabarnett'
components = ['IO']
creation = <Date 2020-11-26.13:47:10.045>
creator = 'jlaurens'
dependencies = []
files = []
hgrepos = []
issue_num = 42473
keywords = []
message_count = 2.0
messages = ['381901', '381905']
nosy_count = 2.0
nosy_names = ['mrabarnett', 'jlaurens']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42473'
versions = ['Python 3.8']

[jlaurens]

Test code:

import re
test='''012345678
         
012345678
'''
pattern = r'^\s+?$'
m = re.search(pattern, test, re.M)
if m:
    print(f'TEST FOUND "{m.span()}"')

    def replace(m):
        print(f'TEST REMOVE {m.span()}')
        return ''

    test = re.sub(pattern, replace, test, re.M)
    m = re.search(pattern, test, re.M)
    if m:
        print(f'TEST STILL THERE "{m.span()}"')

print('COMPILE PATTERN FIRST')
pattern_re = re.compile(pattern, re.M)
m = re.search(pattern_re, test)
if m:
    print(f'TEST FOUND "{m.span()}"')

    def replace(m):
        print(f'TEST REMOVE {m.span()}')
        return ''

    test = re.sub(pattern_re, replace, test)
    m = re.search(pattern_re, test)
    if m:
        print(f'TEST STILL THERE "{m.span()}"')

Actual output:

TEST FOUND "(10, 19)"
TEST STILL THERE "(10, 19)"
COMPILE PATTERN FIRST
TEST FOUND "(10, 19)"
TEST REMOVE (10, 19)

This is an inconsistency between re.search and re.sub. Either this is a bug in the code or in the documentation.

[mrabarnett]

Not a bug.

Argument 4 of re.sub is the count:

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

not the flags.