To avoid infecting the dependency graph with potentially unwanted regex features. If the user wants extra regex features, they should enable them in their project (e.g. just add dependency on "regex" with default features enabled).
A major change like this should be discussed in an issue first. I find issue work better for exploring the problem and solution space while PRs are better for discussing the implementation.
To avoid infecting the dependency graph with potentially unwanted regex features. If the user wants extra regex features, they should enable them in their project (e.g. just add dependency on "regex" with default features enabled).