Here is a sweep of potential mistakes in regexp strings in elisp code in the Emacs tree: obsolete syntax, questionable constructs, and so on.

Most of the warnings are harmless, but some look like outright bugs. Perhaps those with knowledge of the code would be interested in taking a look. Some of the cases are quite entertaining.

The errors were gathered by the rather hacky trawl.el, attached (in case you want to run it on other code). It only requires xr (1.4 or later). To use, run trawl-directory or trawl-file.