# commonPrefix(with:options:) Returns a string containing characters this string and the given string have in common, starting from the beginning of each up to the first characters that aren’t equivalent. ``` func commonPrefix(with aString: T, options: String.CompareOptions = []) -> String where T : StringProtocol ``` --- 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)