how to test if a string DON'T match using protractor

It is definitely worth looking at the API docs. I have these open pretty much all the time. There are lots of Web Driver functions you can use like isEnabled(), isDisplayed(), isSelected() etc. Protractor uses Jasmine syntax so you can use '.toBe(false)' to assert things are false. To check for classes, you can do something like:

expect(myElement.getAttribute('class')).toContain('my-class-name');

To compare strings and assert that they do NOT match you could use .not. Jasmine docs say:

Every matcher's criteria can be inverted by prepending .not:

expect(x).not.toEqual(y); compares objects or primitives x and y and passes if they are not equivalent

I'm using the following to check for NOT matching:

expect(element('legend').text() === "LOGIN_CONNECT").toBe(false);

You can use something like:

expect(model.getText()).not.toContain('abcdef');

There is a .not property nowadays.