为什么Python中的正则表达式“ \\ bpattern \\ b”行为不一致？

Question

I am using Python 3 to demonstrate. There is an example string:

a = "learning is learn and elearn"

s = "@wen is @ and wen@"

I want to do exact match of "learn" and "@", ie, not extracting learning (or @wen) or elearn (or wen@). Therefore, I should get 'learn' and '@'.

re.findall(r'\blearn\b', a) # works
['learn']

or

re.sub(r'\blearn\b', 'z', a) # works
'learning is z and elearn'


re.findall(r'\b@\b', s) # not working
[]

or

re.sub(r'\b@\b', 'z', s) # not working
'@wen is @ and wen@'

Answer 1

From the docs :

\\b Matches the empty string, but only at the beginning or end of a word. A word is defined as a sequence of alphanumeric or underscore characters, so the end of a word is indicated by whitespace or a non-alphanumeric, non-underscore character. Note that formally, \\b is defined as the boundary between a \\w and a \\W character (or vice versa), or between \\w and the beginning/end of the string

In your example, @ is a nonalphanumeric (and non-underscore) character surrounded by other nonalphanumeric characters. Because there are no word characters, there is no word boundary, so \\b will not match.