Rename files using sed and mv

Without a for loop.

ls | grep 'jpg$' | sed '
#Save the original filename
h
#Do the replacement
s/img_\(.*\)-\(.*\)-\(.*\)_\([0-9][0-9]\)\./newyears_20\3-\1-\2_0\4.//
s/img_\(.*\)-\(.*\)-\(.*\)_\([0-9][0-9][0-9]\)/newyears_20\3-\1-\2_\4//
#Bring the original filename back
x
G
s/^\(.*\)\n\(.*\)$/mv "\1" "\2"' | bash

Ommit piping to bash to see the results before mv

Thanks to http://www.gnu.org/software/sed/manual/sed.html#Rename-files-to-lower-case

You can try this script in bash:

for filename in *.jpg; do
  newFilename=$(sed -E 's#img_([0-9]{1,2})-([0-9]{1,2})-([0-9]{1,2})_(.*)$#newyears_20\3-\2-\1_\4#' <<< "$filename")
  mv "$filename" "$newFilename"
done

sed -E is supported by gnu sed also.