Swift remove ONLY trailing spaces from string

A quite simple solution is regular expression, the pattern is one or more(+) whitespace characters(\s) at the end of the string($)

let string = " keep my left side "
let cleansed = string.replacingOccurrences(of: "\\s+$", 
                                         with: "", 
                                      options: .regularExpression)

You can use the rangeOfCharacter function on string with a characterSet. This extension then uses recursion of there are multiple spaces to trim. This will be efficient if you only usually have a small number of spaces.

extension String {
    func trailingTrim(_ characterSet : CharacterSet) -> String {
        if let range = rangeOfCharacter(from: characterSet, options: [.anchored, .backwards]) {
            return self.substring(to: range.lowerBound).trailingTrim(characterSet)
        }
        return self
    }
}

---

"1234 ".trailingTrim(.whitespaces)

returns

"1234"