How to check what a String starts with (prefix) or ends with (suffix) in Swift

Question

I\'m trying to test if a Swift string starts or ends with a certain value. These methods do not exist:

var str = \"Hello, playground\"
str.startsWith(\"Hello         


        

Answer 1

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)) // Hello
let 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, playground
let 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)) // Hello

let 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]) // Hello

let suffixIndex = str.index(str.endIndex, offsetBy: -6)
let suffix = String(str[suffixIndex...]) // ground

Further Reading

• Strings and Characters documentation

Answer 2

Prefix and Suffix Equality

To check whether a string has a particular string prefix or suffix, call the string’s hasPrefix(:) and hasSuffix(:) methods, both of which take a single argument of type String and return a Boolean value.

Apple Doc