Rename files by reordering existing data (regular expression capturing submatches) (See related posts)

Takes all files in a directory that end with .jpg and that have a filename with format DD_MM_YY-HHMM.jpg and renames them as YYMMDDHHMM.jpg

#alter the regex to suit your needs, each () will return a submatch
my $regex = qr!(\d\d)[-](\d\d)[-](\d\d)[_](\d\d\d\d)!;

while(<*.jpg>) {
        $oldname = $_;
        if ($oldname=~m/$regex/) {

                #$1,$2,etc are submatchs
                rename $oldname,$3.$2.$1.$4.".jpg";
        }
}