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
Duncan Booth
> 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.
>
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.
Try:
p.match(' word ', 1).group(0)
and you'll see that you are only match the word, not the surrounding
puctuation.
andrew cooke
wrote:
> andrew cooke <and...@acooke.org> wrote:
> > 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 :-)