host:prompt> perl -MYape::Regex::Explain -le " my $p = YAPE::Regex::Explain->new(qr{^KG\d{3}[JMA]$}); print $p->explain();" 
The regular expression:

(?-imsx:^KG\d{3}[JMA]$)

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):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  KG                       'KG'
----------------------------------------------------------------------
  \d{3}                    digits (0-9) (3 times)
----------------------------------------------------------------------
  [JMA]                    any character of: 'J', 'M', 'A'
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------