use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(
'\b\$AVG'
)->explain;
__END__
The regular expression:

(?-imsx:\b\$AVG)

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
----------------------------------------------------------------------
  \$                       '$'
----------------------------------------------------------------------
  AVG                      'AVG'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------