FAQ 6.18 Why don't word-boundary searches with "\b" work for me?
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6.18: Why don't word-boundary searches with "\b" work for me?
(contributed by brian d foy)
Ensure that you know what \b really does: it's the boundary between a
word character, \w, and something that isn't a word character. That
thing that isn't a word character might be \W, but it can also be the
start or end of the string.
It's not (not!) the boundary between whitespace and non-whitespace, and
it's not the stuff between words we use to create sentences.
In regex speak, a word boundary (\b) is a "zero width assertion",
meaning that it doesn't represent a character in the string, but a
condition at a certain position.
For the regular expression, /\bPerl\b/, there has to be a word boundary
before the "P" and after the "l". As long as something other than a word
character precedes the "P" and succeeds the "l", the pattern will match.
These strings match /\bPerl\b/.
"Perl" # no word char before P or after l
"Perl " # same as previous (space is not a word char)
"'Perl'" # the ' char is not a word char
"Perl's" # no word char before P, non-word char after "l"
These strings do not match /\bPerl\b/.
"Perl_" # _ is a word char!
"Perler" # no word char before P, but one after l
You don't have to use \b to match words though. You can look for
non-word characters surrounded by word characters. These strings match
the pattern /\b'\b/.
"don't" # the ' char is surrounded by "n" and "t"
"qep'a'" # the ' char is surrounded by "p" and "a"
These strings do not match /\b'\b/.
"foo'" # there is no word char after non-word '
You can also use the complement of \b, \B, to specify that there should
not be a word boundary.
In the pattern /\Bam\B/, there must be a word character before the "a"
and after the "m". These patterns match /\Bam\B/:
"llama" # "am" surrounded by word chars
"Samuel" # same
These strings do not match /\Bam\B/
"Sam" # no word boundary before "a", but one after "m"
"I am Sam" # "am" surrounded by non-word chars
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RedGrittyBrick
PerlFAQ Server wrote:
> You can also use the complement of \b, \B, to specify that there should
> not be a word boundary.
>
> In the pattern /\Bam\B/, there must be a word character before the "a"
> and after the "m". These patterns match /\Bam\B/:
>
> "llama" # "am" surrounded by word chars
> "Samuel" # same
>
> These strings do not match /\Bam\B/
>
> "Sam" # no word boundary before "a", but one after "m"
> "I am Sam" # "am" surrounded by non-word chars
>
If /\Bam\B/ differs from /\wam\w/ maybe an example could be added to
illustrate this. If not, perhaps there is a better example of the use of \B?
--
RGB
John W. Krahn
/\Bam\B/ matches two characters while /\wam\w/ matches four characters.
$ perl -le'$_ = "Samuel"; s/\Bam\B/ex/; print'
Sexuel
$ perl -le'$_ = "Samuel"; s/\wam\w/ex/; print'
exel
John
--
Perl isn't a toolbox, but a small machine shop where you
can special-order certain sorts of tools at low cost and
in short order. -- Larry Wall
RedGrittyBrick
John W. Krahn wrote:
> RedGrittyBrick wrote:
>>
>> PerlFAQ Server wrote:
>>
>>> You can also use the complement of \b, \B, to specify that there
>>> should
>>> not be a word boundary.
>>>
>>> In the pattern /\Bam\B/, there must be a word character before
>>> the "a"
>>> and after the "m". These patterns match /\Bam\B/:
>>>
>>> "llama" # "am" surrounded by word chars
>>> "Samuel" # same
>>>
>>> These strings do not match /\Bam\B/
>>>
>>> "Sam" # no word boundary before "a", but one after "m"
>>> "I am Sam" # "am" surrounded by non-word chars
>>
>> If /\Bam\B/ differs from /\wam\w/ maybe an example could be added to
>> illustrate this. If not, perhaps there is a better example of the use
>> of \B?
>
> /\Bam\B/ matches two characters while /\wam\w/ matches four characters.
>
> $ perl -le'$_ = "Samuel"; s/\Bam\B/ex/; print'
> Sexuel
> $ perl -le'$_ = "Samuel"; s/\wam\w/ex/; print'
> exel
>
Yes. I now realise my earlier suggestion is not relevant to this
particular FAQ. I guess some other FAQ or perldoc clarifies when one
might want to use \B and when \w.
However I do have one suggestion for FAQ 6.18: The current version has
these two assertions:
"These patterns match /\Bam\B/:"
"These strings do not match /\Bam\B/"
I suggest, for consistency, the word "patterns" in the first assertion
be replaced by "strings" (as in the second fragment).
--
RGB
Jürgen Exner
>However I do have one suggestion for FAQ 6.18: The current version has
>these two assertions:
>
> "These patterns match /\Bam\B/:"
> "These strings do not match /\Bam\B/"
>
>I suggest, for consistency, the word "patterns" in the first assertion
>be replaced by "strings" (as in the second fragment).
And in addition it is the other way round (the pattern is the subject
and the string the object):
"/\Bam\B/ matches these strings:"
"/\Bam\B/ does not match these strings:"
or
"These strings are being match by /\Bam\B/:"
"These strings are not being match by /\Bam\B/"
jue
brian d foy
<RedGrit...@spamweary.invalid> wrote:
> Yes. I now realise my earlier suggestion is not relevant to this
> particular FAQ. I guess some other FAQ or perldoc clarifies when one
> might want to use \B and when \w.
As with any feature, use the one that does what you need.
Hans Mulder
> RedGrittyBrick wrote:
>> PerlFAQ Server wrote:
>>> You can also use the complement of \b, \B, to specify that there
>>> should
>>> not be a word boundary.
>>> In the pattern /\Bam\B/, there must be a word character before
>>> the "a"
>>> and after the "m". These patterns match /\Bam\B/:
>>> "llama" # "am" surrounded by word chars
>>> "Samuel" # same
>>> These strings do not match /\Bam\B/
>>> "Sam" # no word boundary before "a", but one after "m"
>>> "I am Sam" # "am" surrounded by non-word chars
>> If /\Bam\B/ differs from /\wam\w/ maybe an example could be added to
>> illustrate this. If not, perhaps there is a better example of the use
>> of \B?
> /\Bam\B/ matches two characters while /\wam\w/ matches four characters.
> $ perl -le'$_ = "Samuel"; s/\Bam\B/ex/; print'
> Sexuel
> $ perl -le'$_ = "Samuel"; s/\wam\w/ex/; print'
> exel
Additionally, \W can match when there's no \w around:
$ perl -lw
$_ = "++"; # Look Ma: no \w!
s/\B/Here/g; # Find matches
print;
__END__
Here+Here+Here
$
Hope this helps,
-- HansM