Rename multiple files by prefix taken from folder names, bash

I have a sequence of folders containing mp4 files (one mp4 in one folder). Folders are numbered sequentially like this example:

.. ..
18. Lesson 18 - Sketching a design to cut out
19. Lesson 19 - How to create a sweep
.. ..

the .mp4 files inside these folders are named, respectively:

.. ..
1. Sketching a design to cut out.mp4
1. Creating a sweep.mp4
.. ...

So all .mp4 file names start with "1." regardless of the folder name. I want to rename all mp4 files with an ascending prefix taken from the folder number, i.e.

18. Sketching a design to cut out.mp4
19. Creating a sweep.mp4
.. ..

for this I want to use the following bash script:

for f in *.mp4; do mv "$f" "$(echo "$f" | sed s/1./MYNUMBER/)"; done

How to specify a sequential prefix taken from folder names instead of 'MYNUMBER'? Folder names and filenames both include whitespace. Possibly, there is another way though.

Solution

Using Perl's `rename` (usable in any OS):

Hierarchy:

tree
.
├── 18. Lesson 18 - Sketching a design to cut out
│   └── 1. Sketching a design to cut out.mp4
└── 19. Lesson 19 - How to create a sweep
    └── 1. Creating a sweep.mp4

2 directories, 2 files

Code:

rename -n 's@\./(\d+\.)[^/]+/\d+\.\s*(.*)@$1 $2@' ./*/*.mp4
rmdir *
ls -1

Output:

'18. Sketching a design to cut out.mp4'
'19. Creating a sweep.mp4'

The regular expression matches as follows: