Does it make sense to apply those same changes to the StringSubject.doesNotMatch method?
assertThat("$test").doesNotMatch("$test")
for example would pass even if it was not intended to by the user.
I can't think of an example where doesNotMatch would fail.
assertThat("test").doesNotMatch("test")
would fail intentionally.

Thanks.

RE: doesNotMatch: Looking at our internal users of doesNotMatch makes me so sad :( So many cases like doesNotMatch("^foo"), which checks only that the string does not exactly match "foo." That is, it does not just search for a match, but rather the whole string must match. But I agree that we can't do anything about that here.

One comment: It seems like users would be unlikely to run into this problem the Pattern overload: If a user writes assertThat(foo).matches(Pattern.compile(bar)), then it should be fairly clear that the user is requesting a regex match. I can see 3 courses of action:

1. Still warn about it, as in your PR. After all, what's the harm?
2. Warn about it, but suggest using matches(Pattern.quote(...)) instead of isEqualTo(...). My thinking is that someone might have wrapped matches in a helper method, something like fooMustMatch(String regex). If a user is stuck calling that method, then that user can't simply switch from matches to isEqualTo, since the call to matches happens inside the method. In that case, the fix would be to quote the expected string into regex form with Pattern.quote. (We do somewhat discourage such helper methods, for this and other reasons, but people do it, and we could help them out.)
3. Add the warning for the String overload only.

Thoughts?

Yeah good catch. If someone uses the Pattern overload they very likely know they are checking with a regex. I think option 2 makes the most sense here.
I changed the error message to:
"If you want an exact equality assertion you can escape your regex with Pattern.quote()."

In any case that will give the user a hint since

expected to match: $foo
but was          : $foo

can be quite confusing at first.
Hope you are fine with that and thanks for reviewing the PR. 👍