Use .toMatch to check that a string matches a regular expression.

For example, you might not know what exactly essayOnTheBestFlavor() returns, but you know it’s a really long string, and the substring grapefruit should be in there somewhere. You can test this with:

describe('an essay on the best flavor', () => {
  test('mentions grapefruit', () => {
    expect(essayOnTheBestFlavor()).toMatch(/grapefruit/);
    expect(essayOnTheBestFlavor()).toMatch(new RegExp('grapefruit'));
  });
});

This matcher also accepts a string, which it will try to match:

describe('grapefruits are healthy', () => {
  test('grapefruits are a fruit', () => {
    expect('grapefruits').toMatch('fruit');
  });
});