$chapter =~ s/\D+//g;   # remove all non-digit characters 
# or perhaps to avoid the /g flag, (I wouldn't code it this way
# because it is overly complex) however:
$chapter  =~ s/^(\D*)(\d+)(\D*)/$2/; # remove all optional non-digit stuff
                                     # before or after the digits
# try the above with "XX546YYY", just "453ZZ" and "AAA123ZZZ77548" as
# cases to probe the limits... what happens if it is not just "11VI"?