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 t

Answer 1

edit/update:

Xcode 11.4 • Swift 5.2 or later

import Foundation

extension StringProtocol {
    func index(of string: S, options: String.CompareOptions = []) -> Index? {
        range(of: string, options: options)?.lowerBound
    }
    func endIndex(of string: S, options: String.CompareOptions = []) -> Index? {
        range(of: string, options: options)?.upperBound
    }
    func indices(of string: S, options: String.CompareOptions = []) -> [Index] {
        ranges(of: string, options: options).map(\.lowerBound)
    }
    func ranges(of string: S, options: String.CompareOptions = []) -> [Range] {
        var result: [Range] = []
        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
    }
}

---

usage:

let str = "abcde"
if let index = str.index(of: "cd") {
    let substring = str[..

---

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}]

---

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"]

---

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"]