你可以尝试

let str = "Das Auto ist blau, das andere ist allerdings blau"
 if let range = str.range(of:"blau") {
     print(str.replacingCharacters(in: range, with:"red"))
 }

extension String {

public func replaceFirstExpression(of pattern:String,
                                   with replacement:String) -> String {
    if let range = self.range(of: pattern){
        return self.replacingCharacters(in: range, with: replacement)
    }else{
        return self
    }
}

}