How to have a firstname validation in iOS swift4?

I have a first name validation to be done in swift4 which is in a UITextField. The thing is that:

1. The Firstname should start with a letter and end with a letter,
2. No trailing and leading whitespace.
3. It can have spaces between them, can contain dot.
4. should be between 7 and 18 characters.

Currently i am having the following validation in the UITextField

public func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    let STRING_ACCEPTABLE_CHARACTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
    switch textField {
    case firstName:
        print("firstname")
        if string == " " {
            return false
        } else {
            let cs = NSCharacterSet(charactersIn: STRING_ACCEPTABLE_CHARACTERS).inverted
            let filtered = string.components(separatedBy: cs).joined(separator: "")
            return (string == filtered)
        }
    }
    return true
}

Any idea how to implement the following conditions. Right now I am disabling the space and only accepts the alphabets.

Any help will be very much appreciated

You can use this func for short and clean way

func isValid(testStr:String) -> Bool {
    guard testStr.count > 7, testStr.count < 18 else { return false }

    let predicateTest = NSPredicate(format: "SELF MATCHES %@", "^(([^ ]?)(^[a-zA-Z].*[a-zA-Z]$)([^ ]?))$")
    return predicateTest.evaluate(with: testStr)
}

Test cases

isValid(testStr: " MyNameIsDahiya") // Invalid, leading space
isValid(testStr: "MyNameIsDahiya ") // Invalid, trailing space
isValid(testStr: " MyNameIsDahiya ") // Invalid, leading & trailing space
isValid(testStr: "1MyNameIsDahiya") // Invalid, Leading num
isValid(testStr: "MyNameIsDahiya1") // Invalid, trailing num
isValid(testStr: "1MyNameIsDahiya1") // Invalid, leading & trailing num
isValid(testStr: "MyName") // Invalid, length below 7
isValid(testStr: "MyNameIsDahiyaBlahhblahh") // Invalid, length above 18

isValid(testStr: "MyNameIsDahiya") // Valid,
isValid(testStr: "Mr. Dahiya") // Valid,

Playground Test

Edit

Try this below regex:

(?mi)^[a-z](?!(?:.*\.){2})(?!(?:.* ){2})(?!.*\.[a-z])[a-z. ]{5,16}[a-z]$

It will validate all the conditions and length also.