Xpath expression issue -

i have div this:

<div class="alert alert-danger">please enter email address</div> 

i want write xpath expression such if div contains please enter email address , class exact equal alert alert-danger should return me element. have xpath "//*[contains(text(),'please enter email address')]"

how add condition returns if class attribute "alert alert-danger". if contains "alert alert-danger hello" should not return i.e should n equal match , not like match

//div[@class="alert alert-danger"][.="please enter email address"]