Something like  MP27203801969209 ('MP' followed by 14 digits) can be matched by
    my $MP_14_d = qr{ MP \d{14} }xms;
and in general, a sequence like  9431641426 can be matched by  \d{n} and other variations on the  {} count quantifier. Please see perlre, perlretut, and perlrequick. (Update: See also Pattern Matching, Regular Expressions, and Parsing in Tutorials)