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]

See perlretut for an introduction to regular expressions.