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In some cases standard re module and third-party regex modules raise exceptions with different error messages.
1. re.match(re.compile('.'), 'A', re.I)
 re: Cannot process flags argument with a compiled pattern
 regex: can't process flags argument with a compiled pattern
2. re.compile('(?P<foo_123')
 re: unterminated name
 regex: missing >
3. re.compile('(?P<foo_123>a)(?P=foo_123')
 re: unterminated name
 regex: missing )
4. regex.sub('(?P<a>x)', r'\g<a', 'xx')
 re: unterminated group name
 regex: missing >
5. re.sub('(?P<a>x)', r'\g<', 'xx')
 re: unterminated group name
 regex: bad group name
6. re.sub('(?P<a>x)', r'\g<a a>', 'xx')
 re: bad character in group name
 regex: bad group name
7. re.sub('(?P<a>x)', r'\g<-1>', 'xx')
 re: negative group number
 regex: bad group name
8. re.compile('(?P<foo_123>a)(?P=!)')
 re: bad character in backref group name '!'
 regex: bad group name
9. re.sub('(?P<a>x)', r'\g', 'xx')
 re: missing group name
 regex: missing <
10. re.compile('a\\')
 re.sub('x', '\\', 'x')
 re: bogus escape (end of line)
 regex: bad escape
11. re.compile(r'1円')
 re: bogus escape: '1円'
 regex: unknown group
12. re.compile('[a-')
 re: unexpected end of regular expression
 regex: bad set
13. re.sub(b'.', 'b', b'c')
 re: expected bytes, bytearray, or an object with the buffer interface, str found
 regex: expected bytes instance, str found
14. re.compile(r'\w', re.UNICODE | re.ASCII)
 re: ASCII and UNICODE flags are incompatible
 regex: ASCII, LOCALE and UNICODE flags are mutually incompatible
15. re.compile('(abc')
 re: unbalanced parenthesis
 regex: missing )
16. re.compile('abc)')
 re: unbalanced parenthesis
 regex: trailing characters in pattern
17. re.compile(r'((.)1円+)')
 re: cannot refer to open group
 regex: can't refer to an open group
Looks as in one case re messages are better, and in other cases regex messages are better. In any case it would be good to unify error messages in both modules.