How to check what a String starts with (prefix) or ends with (suffix) in Swift
Updated for Swift 4
Checking what a String starts with and ends with
You can use the hasPrefix(_:)
and hasSuffix(_:)
methods to test equality with another String.
let str = "Hello, playground"if str.hasPrefix("Hello") { // true print("Prefix exists")}if str.hasSuffix("ground") { // true print("Suffix exists")}
Getting the Actual Prefix and Suffix Substrings
In order to get the actual prefix or suffix substring, you can use one of the following methods. I recommend the first method for it's simplicity. All methods use str
as
let str = "Hello, playground"
Method 1: (Recommended) prefix(Int)
and suffix(Int)
let prefix = String(str.prefix(5)) // Hellolet suffix = String(str.suffix(6)) // ground
This is the better method in my opinion. Unlike the methods 2 and 3 below, this method will not crash if the indexes go out of bounds. It will just return all the characters in the string.
let prefix = String(str.prefix(225)) // Hello, playgroundlet suffix = String(str.suffix(623)) // Hello, playground
Of course, sometimes crashes are good because they let you know there is a problem with your code. So consider the second method below as well. It will throw an error if the index goes out of bounds.
Method 2: prefix(upto:)
and suffix(from:)
Swift String indexes are tricky because they have to take into account special characters (like emoji). However once you get the index it is easy to get the prefix or suffix. (See my other answer on String.Index
.)
let prefixIndex = str.index(str.startIndex, offsetBy: 5)let prefix = String(str.prefix(upTo: prefixIndex)) // Hellolet suffixIndex = str.index(str.endIndex, offsetBy: -6)let suffix = String(str.suffix(from: suffixIndex)) // ground
If you want to guard against going out of bounds, you can make an index using limitedBy
(again, see this answer).
Method 3: subscripts
Since String is a collection, you can use subscripts to get the prefix and suffix.
let prefixIndex = str.index(str.startIndex, offsetBy: 5)let prefix = String(str[..<prefixIndex]) // Hellolet suffixIndex = str.index(str.endIndex, offsetBy: -6)let suffix = String(str[suffixIndex...]) // ground