regex - Why doesn't Python's `re.split()` split on zero-length matches?

Question

One particular quirk of the (otherwise quite powerful) re module in Python is that re.split() will never split a string on a zero-length match, for example if I want to split a string along word boundaries:

>>> re.split(r"\s+|\b", "Split along words, preserve punctuation!")
['Split', 'along', 'words,', 'preserve', 'punctuation!']

instead of

['', 'Split', 'along', 'words', ',', 'preserve', 'punctuation', '!']

Why does it have this limitation? Is it by design? Do other regex flavors behave like this?

Answer 1

It's a design decision that was made, and could have gone either way. Tim Peters made this post to explain:

For example, if you split "abc" by the pattern x*, what do you expect? The pattern matches (with length 0) at 4 places, but I bet most people would be surprised to get

['', 'a', 'b', 'c', '']

back instead of (as they do get)

['abc']

Some others disagree with him though. Guido van Rossum doesn't want it changed due to backwards compatibility issues. He did say:

I'm okay with adding a flag to enable this behavior though.

Edit:

There is a workaround posted by Jan Burgy:

>>> s = "Split along words, preserve punctuation!"
>>> re.sub(r"\s+|\b", '\f', s).split('\f')
['', 'Split', 'along', 'words', ',', 'preserve', 'punctuation', '!']

Where '\f' can be replaced by any unused character.

Answer 2

To workaround this problem, you can use the VERSION1 mode of the regex package which makes split() produce zero-length matches as well:

>>> import regex as re
>>> re.split(r"\s+|\b", "Split along words, preserve punctuation!", flags=re.V1)
['', 'Split', 'along', 'words', ',', 'preserve', 'punctuation', '!']

Answer 3

Python supports this as of 3.7:

>>> s = "You! Are you Tom? I am Danny."
>>> re.split(r'(?<=[.!\?])', s)
['You!', ' Are you Tom?', ' I am Danny.', '']

Answer 4

Basically, split() is two different functions into one. If you provide a parameter, it behaves very differently than when called without one.

At first, it would seems that

s.split() == s.split(' \t\n')

but this is not the case, as you have shown. The doc says:

[...] If sep is not specified or is None, any whitespace string is a separator and empty strings are removed from the result. [...]

Even adding a 'remove_empty' parameter it would still behave weird, because the default of 'remove_empty' depends on the 'sep' parameter being there.