Answer by ghostdog74 for Capturing Groups From a Grep RegEx

if you have bash, you can use extended globbing

shopt -s extglob
shopt -s nullglob
shopt -s nocaseglob
for file in +([0-9])_+([a-z])_+([a-z0-9]).jpg
do
   IFS="_"
   set -- $file
   echo "This is your captured output : $2"
done

or

ls +([0-9])_+([a-z])_+([a-z0-9]).jpg | while read file
do
   IFS="_"
   set -- $file
   echo "This is your captured output : $2"
done