Introduce a method to concatenate regex patterns

[aleskva]

BPO	33658
Nosy	@ezio-melotti, @serhiy-storchaka

^{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 = None
created_at = <Date 2018-05-27.08:53:27.253>
labels = ['expert-regex', 'type-feature', '3.7']
title = 'Introduce a method to concatenate regex patterns'
updated_at = <Date 2018-05-28.08:16:03.689>
user = 'https://bugs.python.org/aleskva'

bugs.python.org fields:

activity = <Date 2018-05-28.08:16:03.689>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Regular Expressions']
creation = <Date 2018-05-27.08:53:27.253>
creator = 'aleskva'
dependencies = []
files = []
hgrepos = []
issue_num = 33658
keywords = []
message_count = 2.0
messages = ['317787', '317829']
nosy_count = 5.0
nosy_names = ['ezio.melotti', 'mrabarnett', 'THRlWiTi', 'serhiy.storchaka', 'aleskva']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue33658'
versions = ['Python 3.7']

[aleskva]

After removing inline flags (deprecated in https://bugs.python.org/issue22493) piping patterns will not work:
pats = [r'(?m)^line.continues$', r'(?s)begin.*?end']
re.compile('|'.join(pats))

Maybe there should be introduced some method to merge patterns (similar to re.escape)?

[serhiy-storchaka]

This is not trivial task because inline flags with global scope can be occurred in any part of the pattern (although using them not at the start of the pattern is deprecated).

But starting with 3.7 you can use inline flags with local scope. This will allow you to use simple string concatenation.

pats = [r'(?m:^line.continues$)', r'(?s:begin.*?end)']