remove_extension() {
  sed -E 's/^(.+)\.[^.]+$/\1/'
}

get_extension() {
  sed -E '/^\.?[^.]+$/d; s/^.+\.([^.]*)$/\1/'
}

ffmpeg_av1_opus() {
  if [ "$#" -lt 4 ]; then
    return
  fi
  ffmpeg -i "$4" -c:v libsvtav1 -preset "$1" -crf "$2" -c:a libopus -vbr:a 1 -b:a "$3" "${5:-"$(echo "$4" | remove_extension)_ffmpeg_av1_$1_$2_opus_$3.mkv"}"
}

ffmpeg_av1_lossless_flac() {
  if [ "$#" -eq 0 ]; then
    return
  fi
  ffmpeg -i "$1" -c:v libsvtav1 -svtav1-params lossless=1 -preset -2 -c:a flac "${2:-"$(echo "$1" | remove_extension)_ffmpeg_av1_lossless_flac.mkv"}"
}

ffmpeg_opus() {
  if [ "$#" -lt 2 ]; then
    return
  fi
  ffmpeg -i "$2" -c:a libopus -vbr:a 1 -b:a "$1" "${3:-"$(echo "$2" | remove_extension)_ffmpeg_opus_$1.opus"}"
}

ffmpeg_flac() {
  if [ "$#" -eq 0 ]; then
    return
  fi
  ffmpeg -i "$1" -c:a flac "${2:-"$(echo "$1" | remove_extension)_ffmpeg_flac.flac"}"
}

ffmpeg_segment() {
  if [ "$#" -lt 2 ]; then
    return
  fi
  ffmpeg -i "$2" -c copy -map 0 -segment_time "$1" -f segment -reset_timestamps 1 "${3:-"$(echo "$2" | remove_extension)_ffmpeg_%04d.$(echo "$2" | get_extension)"}"
}

ffmpeg_concat_auto() {
  local files=()
  if [ "$#" -eq 0 ]; then
    for f in *; do
      test -f "$f" && files+=("$f")
    done
  else
    files=("$@")
  fi
  ffmpeg -f concat -safe 0 -i <(printf "file '$PWD/%s'\n" "${files[@]}") -c copy "$(echo "${files[0]}" | remove_extension | sed -E 's/_ffmpeg_[0-9]+$//').$(echo "${files[0]}" | get_extension)"
}

ffmpeg_concat() {
  if [ "$#" -lt 2 ]; then
    return
  fi
  ffmpeg -f concat -safe 0 -i <(printf "file '$PWD/%s'\n" "${@:2}") -c copy "$1"
}

echo_non_ffmpeg() {
  while IFS= read -r -d '' f; do
    case "$f" in
      *ffmpeg_*) ;;
      *)
        echo "$f"
        ;;
    esac
  done < <(find . -type f -print0)
}

remove_non_ffmpeg() {
  while IFS= read -r -d '' f; do
    case "$f" in
      *ffmpeg_*) ;;
      *)
        rm -f "$f"
        ;;
    esac
  done < <(find . -type f -print0)
}

echo_ffmpeg() {
  while IFS= read -r -d '' f; do
    case "$f" in
      *ffmpeg_*)
        echo "$f"
        ;;
      *) ;;
    esac
  done < <(find . -type f -print0)
}

remove_ffmpeg() {
  while IFS= read -r -d '' f; do
    case "$f" in
      *ffmpeg_*)
        rm -f "$f"
        ;;
      *) ;;
    esac
  done < <(find . -type f -print0)
}

mv_space_underscore() {
  while IFS= read -r -d '' f; do
    new="${f// /_}"
    [[ -e "$new" ]] && {
      echo "skipping '$f', '$new' exists"
      continue
    }
    mv -- "$f" "$new"
  done < <(find . -depth -name '* *' -print0)
}

mv_space_hyphen() {
  while IFS= read -r -d '' f; do
    new="${f// /-}"
    [[ -e "$new" ]] && {
      echo "skipping '$f', '$new' exists"
      continue
    }
    mv -- "$f" "$new"
  done < <(find . -depth -name '* *' -print0)
}

Edit: Add -vbr:a 1 to opus, thanks to exdor.