my $FOUR_DIGIT_CAPTURE 
    = qr/
        ( 
          # left as an exercise
        )
        /x;
while (<DATA>) {
    if (my ($four_digit_num) = /$FOUR_DIGIT_CAPTURE/) {
        print # left as an exercise
    }
    else {
        # left as an exercise
    }
}
__DATA__
j9999120.125
Be sure to include lots of interesting test cases.