Help with Regexp, \b

[andrew cooke]

This is a bit embarassing, but I seem to be misunderstanding how \b
works in regexps.

Please can someone explain why the following fails:

from re import compile

p = compile(r'\bword\b')
m = p.match(' word ')
assert m

My understanding is that \b matches a space at the start or end of a
word, and that "word" is a word - http://docs.python.org/library/re.html

What am I missing here? I suspect I am doing something very stupid.

Thanks,
Andrew

 

[andrew cooke]

Please can someone explain why the following fails:

        from re import compile

        p = compile(r'\bword\b')
        m = p.match(' word ')
        assert m

[...]
You misunderstand what \b does: it doesn't match a space, it matches a 0
length string on a boundary between a non-word and a word.

[...]

That's what I thought it did... Then I read the docs and confused
"empty string" with "space"(!) and convinced myself otherwise. I
think I am going senile.

Thanks very much!
Andrew

 

[John Machin]

That's what I thought it did...  Then I read the docs and confused
"empty string" with "space"(!) and convinced myself otherwise.  I
think I am going senile.

Not necessarily. Conflating concepts like "string containing
whitespace", "string containing space(s)", "empty aka 0-length
string", None, (ASCII) NUL, and (SQL) NULL appears to be an age-
independent problem