Introduce a method to concatenate regex patterns

[e2a53e08-d9ff-4ce2-9a94-6b72883af0fc]

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

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['expert-regex', 'type-feature', '3.7'] title = 'Introduce a method to concatenate regex patterns' updated_at = user = 'https://bugs.python.org/aleskva' ``` bugs.python.org fields: ```python activity = actor = 'serhiy.storchaka' assignee = 'none' closed = False closed_date = None closer = None components = ['Regular Expressions'] creation = 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'] ```

[e2a53e08-d9ff-4ce2-9a94-6b72883af0fc]

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)']