re.sub（）文档误解

Question

I've just started learning regular expressions and the documentation for re.sub() states:

Changed in version 3.5 : Unmatched groups are replaced with an empty string.

Deprecated since version 3.5, will be removed in version 3.6 : Unknown escapes consist of '\\' and ASCII letter now raise a deprecation warning and will be forbidden in Python 3.6.

Is re.sub() deprecated? What should I use then?

Answer 1

You misunderstand the documentation. The re.sub() function is not deprecated . The deprecation warning concerns specific syntax .

Earlier in the re.sub() documentation you'll find this:

Unknown escapes such as \\& are left alone.

If you used and unknown escape with an ASCII letter the escape will no longer be ignored, you'll get a warning instead. This applies both to re.sub() replacement patterns and to the regular expression patterns. The same warning is placed in the section on regex pattern syntax.

The Changed in version 3.5 line also concerns how re.sub() works. Rather than raise an exception when there is no matching group for a \\number backreference, an empty string is inserted at that location.

Answer 2

The two entries are not related, and re.sub will not be deprecated.

In Python version earlier than 3.5 re.sub failed if a backreference was used to a capturing group that did not participate in the match. See Empty string instead of unmatched group error SO question.

An example where the failure occurred:

import re
old = 'regexregex'
new = re.sub(r'regex(group)?regex', r'something\1something', old)
print(new) # => fail as there is no "group" in between "regex" and "regex" in "regexregex"
#    and Group 1 was not initialized with an empty string, i.e. remains null

As for the second one, it only says that there will be a warning (and later forbidden) if you use an unknown for a regex engine literal backslash followed with an ASCII character. The backslash was just ignored in them before, in Python 2.x through 3.5, print(re.sub(r'\\j', '', 'joy')) prints oy . So, these will be forbidden in Python 3.6.