@KevinH is right, of course, but I will be the last person to discourage people from writing their own plugins.
My advice:
1. First of all, read the plugin documentation, especially the first chapter titled "The Anatomy of a Plugin".
Link:
https://github.com/Sigil-Ebook/Sigil...ork_rev15.epub
2. Using regex for this purpose is a bit dangerous. The best would be a CSV file as simple as possible: image file name, alternative text.
3. You will already find the key functions in the Access-Aide plugin.
4. Using Python functions is indeed tempting, although I think I would prefer a plugin.
5. I would be happy to test a working version.