handling of zero length matches has changed with python 3.7. Consider the following with python 3.6 (and previous):
>>> import re
>>> print(re.sub('a*', 'x', 'bac'))
xbxcx
>>> print(re.sub('.*', 'x', 'bac'))
x
We get the following with python 3.7:
>>> import re
>>> print(re.sub('a*', 'x', 'bac'))
xbxxcx
>>> print(re.sub('.*', 'x', 'bac'))
xx
I understand this is the standard behavior of PCRE. Furthermore, re.finditer() seems to have always detected the additional match:
>>> for m in re.finditer('a*', 'bac'):
... print(m.start(0), m.end(0), m.group(0))
...
0 0
1 2 a
2 2
3 3
That said, I'm interested in retrieving the behavior of python 3.6 (this is for a hobby project implementing sed in python).
I can come with the following solution:
def sub36(regex, replacement, string):
compiled = re.compile(regex)
class Match(object):
def __init__(self):
self.prevmatch = None
def __call__(self, match):
try:
if match.group(0) == '' and self.prevmatch and match.start(0) == self.prevmatch.end(0):
return ''
else:
return re._expand(compiled, match, replacement)
finally:
self.prevmatch = match
return compiled.sub(Match(), string)
which gives:
>>> print(re.sub('a*', 'x', 'bac'))
xbxxcx
>>> print(sub36('a*', 'x', 'bac'))
xbxcx
>>> print(re.sub('.*', 'x', 'bac'))
xx
>>> print(sub36('.*', 'x', 'bac'))
x
However, this seems very crafted for these examples.
What would be the right way to implement python 3.6 behavior for re.sub() zero length matches with python 3.7?
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