if the function is supposed to return a boolean (i.e. false or true), then isValidEmail seems like the most expressive, in that it leaves the least room for doubt regarding what the method requires as input and what it will return as output
if the function is supposed to throw an exception when the string isn't a valid email address and do nothing if it is a valid email address, then validateEmail seems like the most expressive choice
My thoughts on the other options:
isEmailValid: this one seems more at home in e.g. user.isEmailValid(), so as a plain function this name adds more ambiguity than is necessary
emailIsValid: the same, with the additional concern that it could just as well be the name of a variable/field than a method to be called/queried, so even more unnecessary ambiguity
isEmail: very, very vague. Does this refer to a valid email address, a known email address, an object of the type Email, the route via which the user prefers to be contacted ... ?
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As with most things, it depends:
isValidEmailseems like the most expressive, in that it leaves the least room for doubt regarding what the method requires as input and what it will return as outputvalidateEmailseems like the most expressive choiceMy thoughts on the other options:
user.isEmailValid(), so as a plain function this name adds more ambiguity than is necessary