use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(qr/\bCA\b/)->explain;
__END__

The regular expression:

(?-imsx:\bCA\b)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  CA                       'CA'
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

$ perl -pe " s/\bCA\b/California/ "
Hi ca  Ca CA dooodie
Hi ca  Ca California dooodie
[download]