set -x

oldname='The New Town Cryer - 01 Oct 2020.pdf'
date_re='(^.*) - ([[:digit:]]{2}) ([[:alpha:]]+) ([[:digit:]]{4})(.*)'
if [[ $oldname =~ $date_re ]]; then
  basename=${BASH_REMATCH[1]}
  day=${BASH_REMATCH[2]}
  month=${BASH_REMATCH[3]}
  year=${BASH_REMATCH[4]}
  ext=${BASH_REMATCH[5]}
  new_date=$(date -d "${day} ${month} ${year}" +%Y-%m-%d)
  newname="${new_date} - ${basename}${ext}"
  echo "Old name: $oldname"
  echo "New name: $newname"
fi