You already asked that question and got a good answer from kennethk at Re: Word boundaries. If you didn't understand the reply you should post a follow up question there.

Hello, sandyago.

In regular expression syntax many flags are paired upper- and lower-case letters, one signifying the opposite of the other.

In this case \b matches a boundary between a word character and a non-word character, while \B matches anything else.

$ perl -E'
for ("cat","a dog") { say if /\ba/; }
'
[download]

a dog
[download]

$ perl -E'
for ("cat","a dog") { say if /\Ba/; }
'
[download]

cat
[download]

G'day sandyago,

Very briefly, \b matches a word boundary and \B matches NOT a word boundary.

Read "perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes". In particular, the Assertions section of that documentation has a fairly in-depth discussion as well as examples.

Finally, there's quite a few examples of the upper- and lower-case versions having an opposite meaning. For example, \d matches a digit and \D matches NOT a digit; similarly for \s and \S, \w and \W and so on.