What does a backslash followed by a single quote mean in a regular expression?

Question

Looking at this stackoverflow answer on an old way for specifying the org-file-apps list, I can see that there is a backslash followed by a single-quote character after each file type extension, e.g. ...pdf\\'"...

What does it mean? What purpose does the backslash plus quote serve?

Answer 1

It's a special construct in emacs regexp that matches the end of a string (not just the end of a line). Quoting the the manual

\'

matches the empty string, but only at the end of the string or buffer (or its accessible portion) being matched against.