# Substring A slice of a string. ``` @frozen struct Substring ``` ## Overview When you create a slice of a string, a `Substring` instance is the result. Operating on substrings is fast and efficient because a substring shares its storage with the original string. The `Substring` type presents the same interface as `String`, so you can avoid or defer any copying of the string’s contents. The following example creates a `greeting` string, and then finds the substring of the first sentence: ``` let greeting = "Hi there! It's nice to meet you! 👋" let endOfSentence = greeting.firstIndex(of: "!")! let firstSentence = greeting[...endOfSentence] // firstSentence == "Hi there!" ``` You can perform many string operations on a substring. Here, we find the length of the first sentence and create an uppercase version. ``` print("'\(firstSentence)' is \(firstSentence.count) characters long.") // Prints "'Hi there!' is 9 characters long." let shoutingSentence = firstSentence.uppercased() // shoutingSentence == "HI THERE!" ``` # Converting a Substring to a String This example defines a `rawData` string with some unstructured data, and then uses the string’s `prefix(while:)` method to create a substring of the numeric prefix: ``` let rawInput = "126 a.b 22219 zzzzzz" let numericPrefix = rawInput.prefix(while: { "0"..."9" ~= $0 }) // numericPrefix is the substring "126" ``` When you need to store a substring or pass it to a function that requires a `String` instance, you can convert it to a `String` by using the `String(_:)` initializer. Calling this initializer copies the contents of the substring to a new string. ``` func parseAndAddOne(_ s: String) -> Int { return Int(s, radix: 10)! + 1 } _ = parseAndAddOne(numericPrefix) // error: cannot convert value... let incrementedPrefix = parseAndAddOne(String(numericPrefix)) // incrementedPrefix == 127 ``` Alternatively, you can convert the function that takes a `String` to one that is generic over the `StringProtocol` protocol. The following code declares a generic version of the `parseAndAddOne(_:)` function: ``` func genericParseAndAddOne(_ s: S) -> Int { return Int(s, radix: 10)! + 1 } let genericallyIncremented = genericParseAndAddOne(numericPrefix) // genericallyIncremented == 127 ``` You can call this generic function with an instance of either `String` or `Substring`. > Important: Don’t store substrings longer than you need them to perform a > specific operation. A substring holds a reference to the entire storage > of the string it comes from, not just to the portion it presents, even > when there is no other reference to the original string. Storing > substrings may, therefore, prolong the lifetime of string data that is > no longer otherwise accessible, which can appear to be memory leakage. ## Relationships ### Conforms To [`Attachable`](/documentation/Testing/Attachable) [`SendableMetatype`](/documentation/Swift/SendableMetatype) [`LosslessStringConvertible`](/documentation/Swift/LosslessStringConvertible) [`ExpressibleByExtendedGraphemeClusterLiteral`](/documentation/Swift/ExpressibleByExtendedGraphemeClusterLiteral) [`Copyable`](/documentation/Swift/Copyable) [`Comparable`](/documentation/Swift/Comparable) [`Escapable`](/documentation/Swift/Escapable) [`BidirectionalCollection`](/documentation/Swift/BidirectionalCollection) [`CustomTestStringConvertible`](/documentation/Testing/CustomTestStringConvertible) [`TextOutputStreamable`](/documentation/Swift/TextOutputStreamable) [`Collection`](/documentation/Swift/Collection) [`ExpressibleByStringLiteral`](/documentation/Swift/ExpressibleByStringLiteral) [`Hashable`](/documentation/Swift/Hashable) [`ExpressibleByStringInterpolation`](/documentation/Swift/ExpressibleByStringInterpolation) [`Sequence`](/documentation/Swift/Sequence) [`TextOutputStream`](/documentation/Swift/TextOutputStream) [`CustomDebugStringConvertible`](/documentation/Swift/CustomDebugStringConvertible) [`CustomStringConvertible`](/documentation/Swift/CustomStringConvertible) [`ExpressibleByUnicodeScalarLiteral`](/documentation/Swift/ExpressibleByUnicodeScalarLiteral) [`StringProtocol`](/documentation/Swift/StringProtocol) [`Equatable`](/documentation/Swift/Equatable) [`CustomReflectable`](/documentation/Swift/CustomReflectable) [`RangeReplaceableCollection`](/documentation/Swift/RangeReplaceableCollection) [`Sendable`](/documentation/Swift/Sendable) [`RegexComponent`](/documentation/Swift/RegexComponent) --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)