Index of a substring in a string with Swift

Question

I'm used to do this in JavaScript:

var domains = "abcde".substring(0, "abcde".indexOf("cd")) // Returns "ab" 

Swift doesn't have this function, how to do something similar?

Answer 1

edit/update:

Xcode 11.4 • Swift 5.2 or later

import Foundation  extension StringProtocol {     func index<S: StringProtocol>(of string: S, options: String.CompareOptions = []) -> Index? {         range(of: string, options: options)?.lowerBound     }     func endIndex<S: StringProtocol>(of string: S, options: String.CompareOptions = []) -> Index? {         range(of: string, options: options)?.upperBound     }     func indices<S: StringProtocol>(of string: S, options: String.CompareOptions = []) -> [Index] {         ranges(of: string, options: options).map(\.lowerBound)     }     func ranges<S: StringProtocol>(of string: S, options: String.CompareOptions = []) -> [Range<Index>] {         var result: [Range<Index>] = []         var startIndex = self.startIndex         while startIndex < endIndex,             let range = self[startIndex...]                 .range(of: string, options: options) {                 result.append(range)                 startIndex = range.lowerBound < range.upperBound ? range.upperBound :                     index(range.lowerBound, offsetBy: 1, limitedBy: endIndex) ?? endIndex         }         return result     } } 

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usage:

let str = "abcde" if let index = str.index(of: "cd") {     let substring = str[..<index]   // ab     let string = String(substring)     print(string)  // "ab\n" } 

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let str = "Hello, playground, playground, playground" str.index(of: "play")      // 7 str.endIndex(of: "play")   // 11 str.indices(of: "play")    // [7, 19, 31] str.ranges(of: "play")     // [{lowerBound 7, upperBound 11}, {lowerBound 19, upperBound 23}, {lowerBound 31, upperBound 35}] 

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case insensitive sample

let query = "Play" let ranges = str.ranges(of: query, options: .caseInsensitive) let matches = ranges.map { str[$0] }   // print(matches)  // ["play", "play", "play"] 

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regular expression sample

let query = "play" let escapedQuery = NSRegularExpression.escapedPattern(for: query) let pattern = "\\b\(escapedQuery)\\w+"  // matches any word that starts with "play" prefix  let ranges = str.ranges(of: pattern, options: .regularExpression) let matches = ranges.map { str[$0] }  print(matches) //  ["playground", "playground", "playground"] 

Answer 2

Using String[Range<String.Index>] subscript you can get the sub string. You need starting index and last index to create the range and you can do it as below

let str = "abcde" if let range = str.range(of: "cd") {   let substring = str[..<range.lowerBound] // or str[str.startIndex..<range.lowerBound]   print(substring)  // Prints ab } else {   print("String not present") } 

If you don't define the start index this operator ..< , it take the starting index. You can also use str[str.startIndex..<range.lowerBound] instead of str[..<range.lowerBound]