How to ensure a string has a substring exactly n times?

You can have the regex like this which test String "myString" has SubString "xyz" n times.

Assertions.assertThat(myString).containsPattern("(xyz.*?){n}");

You can replace n with the count of substring you want to test.

You are probably looking for StringUtils.countMatches

Counts the number of occurrences of one String in another