Swift 4.
let abc = "Hello world"
let result = abc.replacingOccurrences(of: " ", with: "_", options: NSString.CompareOptions.literal, range:nil)
print(result :(result))
//output =
let abc = "Hello world" let result = abc.replacingOccurrences(of: " ", with: "_", options: NSString.CompareOptions.literal, range:nil) print(result :\(result)) result Output: Hello_world
result : Hello_world