I'm learning Python's regular expression, following is working as I expected:
>>> import re
>>> re.split('\s+|:', 'find a str:s2')
['find', 'a', 'str', 's2']
But when I change + to *, the output is weird to me:
>>> re.split('\s*|:', 'find a str:s2')
['find', 'a', 'str:s2']
How is such pattern interpreted in Python?
The 'side effect' you are seeing is that re.split() will only split on matches that are longer than 0 characters.
The \s*|: pattern matches either on zero or more spaces, or on :, whichever comes first. But zero spaces matches everywhere. In those locations where more than zero spaces matched, the split is made.
Because the \s* pattern matches every time a character is considered for splitting, the next option : is never considered.
Splitting on non-empty matches is called out explicitly in the re.split() documentation:
Note that split will never split a string on an empty pattern match.
If you reverse the pattern, : is considered, as it is the first choice:
>>> re.split(':|\s*', 'find a str:s2')
['find', 'a', 'str', 's2']
If you meant to do "or" for your matching, then you have to do something like this:
re.split('(\s*|:)', 'find a str:s2')
In short:
"+" means "at least one character".
"*" any (or none)
