Added and amended to use pure Swift where possible

Common Swift String Extensions

import Foundation

extension String
{

// Works in Xcode but not Playgrounds because of a bug with .insert()

mutating func insertString(string:String,ind:Int) {
var insertIndex = advance(self.startIndex, ind, self.endIndex)
for c in string {
self.insert(c, atIndex: insertIndex)
insertIndex = advance(insertIndex, 1)
}
}

// new replace method using replaceRange (replaces all instances of string)
  mutating func replace(string:String, replacement:String) {
        

        let ranges = self.rangesOfString(string)
        // if the string isn't found return unchanged string
        
      
        for r in ranges {
          self.replaceRange(r, with: replacement)
            }
        }
        
// Swift pure containsString

func containsString(findStr:String) -> Bool {
        var arr = [Range<String.Index>]()
        var startInd = self.startIndex
        var i = 0
        // test first of all whether the string is likely to appear at all
        if contains(self, first(findStr)!) {
            startInd = find(self,first(findStr)!)!
        }
        else {
            return false
        }
        // set starting point for search based on the finding of the first character
        i = distance(self.startIndex, startInd)
        while i<=countElements(self)-countElements(findStr) {
            if self[advance(self.startIndex, i)..<advance(self.startIndex, i+countElements(findStr))] == findStr {
                arr.append(Range(start:advance(self.startIndex, i),end:advance(self.startIndex, i+countElements(findStr))))
               return true
            }
            i++
        }
        return false
    }


// pure Swift - removeAtIndex no longer required, since the addition of following methods to String type
// mutating func removeAtIndex(i: String.Index) -> Character
// mutating func removeRange(subRange: Range<String.Index>)
// mutating func removeAll(keepCapacity: Bool = default)


// insert() method written in pure Swift, overloads the new String type method of the same name 

func insert(string:String,ind:Int) -> String {

var insertIndex = advance(self.startIndex, ind, self.endIndex)
var returnString = toString(self)
for c in string {
returnString.insert(c, atIndex: insertIndex)
insertIndex = advance(insertIndex, 1)
}
return returnString
}
    
// rangesOfString: written in pure Swift (no Cocoa)
func rangesOfString(findStr:String) -> [Range<String.Index>] {
var arr = [Range<String.Index>]()
var startInd = self.startIndex
// check first that the first character of search string exists
if contains(self, first(findStr)!) {
    // if so set this as the place to start searching
    startInd = find(self,first(findStr)!)!
}
else {
    // if not return empty array
    return arr
    }
var i = distance(self.startIndex, startInd)
while i<=countElements(self)-countElements(findStr) {
    if self[advance(self.startIndex, i)..<advance(self.startIndex, i+countElements(findStr))] == findStr {
        arr.append(Range(start:advance(self.startIndex, i),end:advance(self.startIndex, i+countElements(findStr))))
        i = i+countElements(findStr)-1
// check again for first occurrence of character (this reduces number of times loop will run
if contains(self[advance(self.startIndex, i)..<self.endIndex], first(findStr)!) {
    // if so set this as the place to start searching
i = distance(self.startIndex,find(self[advance(self.startIndex, i)..<self.endIndex],first(findStr)!)!) + i
countElements(findStr)
}
else {
return arr
}

    }
i++
}
    return arr
}


func stringByReplacingOccurrencesOfString(string:String, replacement:String) -> String {
        
        // get ranges first using rangesOfString: method, then glue together the string using ranges of existing string and old string
        
        let ranges = self.rangesOfString(string)
        // if the string isn't found return unchanged string
        if ranges.isEmpty {
            return self
        }
        // using toString to make a copy so that self isn't altered
        var newString = toString(self)
 for r in ranges {
          newString.replaceRange(r, with: replacement)
            }
        return newString
    }
    
// Added String splitting methods that return arrays (Pure Swift)
  func splitStringByCharacters() -> [Character] {
        return map(self){return $0}
    }
    
    func splitStringByLines() -> [String] {
        return split(self, {contains("\u{2028}\n\r", $0)
            }, allowEmptySlices: false)
    }
    
    func splitStringByWords() -> [String] {
        return split(self, {contains(" .,!:;()[]{}<>?\"'\u{2028}\u{2029}\n\r", $0)}, allowEmptySlices: false)
    }
    
    func splitStringByParagraphs() -> [String] {
        return split(self, {contains("\u{2029}\n\r", $0)
            }, allowEmptySlices: false)
        
    }

        func splitStringBySentences() -> [String] {
        let arr:[Character] = ["\u{2026}",".","?", "!"]
        var startInd = self.startIndex
        var strArr = [String]()
        for b in enumerate(self) {
            for a in arr {
                if a == b.element {
                    
                    var endInd = advance(self.startIndex,b.index,self.endIndex)
                    
                    //TODO: add method to allow for multiple punctuation at end of sentence, e.g. ??? or !!!
    
                    var str = self[startInd...endInd]
                    
                    // removes initial spaces and returns from sentence
                    if contains(" \u{2028}\u{2029}\n\r",first(str)!)  {
                        str = dropFirst(str)
                    }
                    strArr.append(str)
                    startInd = advance(endInd,1,self.endIndex)

                }
    
            }
        
        }
    return strArr
        
    }
    
 func regexMatchesInString(regexString:String) -> [String] {
        var arr = [String]()
        var rang = startIndex..<endIndex
        var foundRange:Range<String.Index>?
        
        repeat
        {
            foundRange = rangeOfString(regexString, options: NSStringCompareOptions.RegularExpressionSearch, range: rang, locale: nil)
            
            if let fR = foundRange {
                arr.append(substringWithRange(fR))
                rang.startIndex = fR.endIndex
            }
        }
            while foundRange != nil
        return arr
    }

    var length: Int {
        get {
            return countElements(self)
        }
    }
    
  
    
  
    
    subscript (i: Int) -> Character
    {
        get {
            let index = advance(startIndex, i)
            return self[index]
        }
    }
    
    subscript (r: Range<Int>) -> String
    {
        get {
            let startIndex = advance(self.startIndex, r.startIndex)
            let endIndex = advance(self.startIndex, r.endIndex - 1)
            
            return self[Range(start: startIndex, end: endIndex)]
        }
    }
    
    func subString(startIndex: Int, length: Int) -> String
    {
        var start = advance(self.startIndex, startIndex)
        var end = advance(self.startIndex, startIndex + length)
        return self.substringWithRange(Range<String.Index>(start: start, end: end))
    }
    
    func indexOf(target: String) -> Int
    {
        var range = self.rangeOfString(target)
        if let range = range {
            return distance(self.startIndex, range.startIndex)
        } else {
            return -1
        }
    }
    
    func indexOf(target: String, startIndex: Int) -> Int
    {
        var startRange = advance(self.startIndex, startIndex)
        
        var range = self.rangeOfString(target, options: NSStringCompareOptions.LiteralSearch, range: Range<String.Index>(start: startRange, end: self.endIndex))
        
        if let range = range {
            return distance(self.startIndex, range.startIndex)
        } else {
            return -1
        }
    }
    
    func lastIndexOf(target: String) -> Int
    {
        var index = -1
        var stepIndex = self.indexOf(target)
        while stepIndex > -1
        {
            index = stepIndex
            if stepIndex + target.length < self.length {
                stepIndex = indexOf(target, startIndex: stepIndex + target.length)
            } else {
                stepIndex = -1
            }
        }
        return index
    }
    
    // Updated isMatch to remove reliance on NSRegularExpression
      func isMatch(regex: String, options: NSStringCompareOptions?) -> Bool
    {
        
        let match = self.rangeOfString(regex, options: options ?? nil, range: Range(start: self.startIndex, end: self.endIndex), locale: nil)
        return match != nil ? true : false
    }
    
    // getMatches updated to remove reliance on NSRegularExpression
    func getMatches(str: String, options: NSStringCompareOptions?) -> [Range<String.Index>] {
        var arr = [Range<String.Index>]()
        var rang = startIndex..<endIndex
        var foundRange:Range<String.Index>?

        repeat
        {
            foundRange = rangeOfString(str, options: options ?? [], range: rang, locale: nil)

            if let fR = foundRange {
                arr.append(fR)
                rang.startIndex = fR.endIndex
            }
        }
            while foundRange != nil
        return arr
    }
    
    private var vowels: [String]
    {
     
        return ["a", "e", "i", "o", "u"]
     
    }
    
    private var consonants: [String]
    {
     
        return ["b", "c", "d", "f", "g", "h", "j", "k", "l", "m", "n", "p", "q", "r", "s", "t", "v", "w", "x", "z"]
     
    }
    
    func pluralize(count: Int) -> String
    {
        if count == 1 {
            return self
        } else {
            var lastChar = self.subString(self.length - 1, length: 1)
            var secondToLastChar = self.subString(self.length - 2, length: 1)
            var prefix = "", suffix = ""
            
            if lastChar.lowercaseString == "y" && vowels.filter({x in x == secondToLastChar}).count == 0 {
                prefix = self[0...self.length - 1]
                suffix = "ies"
            } else if lastChar.lowercaseString == "s" || (lastChar.lowercaseString == "o" && consonants.filter({x in x == secondToLastChar}).count > 0) {
                prefix = self[0...self.length]
                suffix = "es"
            } else {
                prefix = self[0...self.length]
                suffix = "s"
            }
            
            return prefix + (lastChar != lastChar.uppercaseString ? suffix : suffix.uppercaseString)
        }
    }
    
    // for fun, not sure whether this has a practical application
    func removeDuplicates(array:String) -> String {
    var arr = array
    var indArr = [Int]()
    var tempArr = arr
    var i = 0
    for a in arr {
        
        if contains(prefix(arr, i), a) {
            indArr.append(i)
        }
        
        i++
    }
    
    
    var ind = 0
    for i in indArr {
        arr.removeAtIndex(i-ind)
        ind++
    }
    return arr
}
}

Usage

// sketchyTech added and updated begins

"Hello".regexMatchesInString("[^Hh]{1,}")
"Hello".isMatch("[x-z]{1,}", options: NSStringCompareOptions.RegularExpressionSearch)
"Hello".getMatches("[x-z]{1,}", options: NSStringCompareOptions.RegularExpressionSearch)

// pure Swift methods
"Hello".splitStringByCharacters()
"Hello. This is a new method.".splitStringByWords()
"Hello. This is a new method.".splitStringBySentences()
"Hello. This is a new method.\n I hope you enjoy using it.".splitStringByParagraphs()

"a very good hello, hello".rangesOfString("hello")
"a very good hello, hello".stringByReplacingOccurrencesOfString("hello",replacement:"bye")
"Hello World!".insert("Swift ", ind: 6)

// update replace method to use new replaceRange method
"a very good hello, hello".replace("hello",replacement:"bye")

// for fun (also pure Swift)
let str = "hello world"
str.removeDuplicates()

// insertString()
var a = "hello world"
a.insertString("big ", ind:6)


REMOVED: removeAtIndex
// pure Swift - removeAtIndex no longer required, since the addition of following methods to String type
// mutating func removeAtIndex(i: String.Index) -> Character
// mutating func removeRange(subRange: Range<String.Index>)
// mutating func removeAll(keepCapacity: Bool = default)

REMOVED: contains() and replaced with containsString()
str.containsString("play")

// sketchyTech added and updated ends



"ReplaceMe".replace("Me", withString: "You") == "ReplaceYou"
"MeReplace".replace("Me", withString: "You") == "YouReplace"
"ReplaceMeNow".replace("Me", withString: "You") == "ReplaceYouNow"

"0123456789"[0] == "0"
"0123456789"[5] == "5"
"0123456789"[9] == "9"

"0123456789"[5...6] == "5"
"0123456789"[0...1] == "0"
"0123456789"[8...9] == "8"
"0123456789"[1...5] == "1234"
"Reply"[0...4] == "Repl"
"Hello, playground"[0...5] == "Hello"
"Coolness"[4...7] == "nes"

"Awesome".indexOf("nothin") == -1
"Awesome".indexOf("Awe") == 0
"Awesome".indexOf("some") == 3
"Awesome".indexOf("e", startIndex: 3) == 6
"Awesome".lastIndexOf("e") == 6
"Cool".lastIndexOf("o") == 2

var emailRegex = "[a-z_\\-\\.]+@[a-z_\\-\\.]{3,}"
"[email protected]".isMatch(emailRegex, options: NSStringCompareOptions.CaseInsensitiveSearch) == true
"email-test.com".isMatch(emailRegex, options: NSStringCompareOptions.InsensitiveSearch) == false

var testText = "[email protected], [email protected], [email protected]"
var matches = testText.getMatches(emailRegex, options: NSStringCompareOptions.InsensitiveSearch)
matches.count == 3
testText.subString(matches[0].range.location, length: matches[0].range.length) == "[email protected]"
testText.subString(matches[1].range.location, length: matches[1].range.length) == "[email protected]"
testText.subString(matches[2].range.location, length: matches[2].range.length) == "[email protected]"

"Reply".pluralize(0) == "Replies"
"Reply".pluralize(1) == "Reply"
"Reply".pluralize(2) == "Replies"
"REPLY".pluralize(3) == "REPLIES"
"Horse".pluralize(2) == "Horses"
"Boy".pluralize(2) == "Boys"
"Cut".pluralize(2) == "Cuts"
"Boss".pluralize(2) == "Bosses"
"Domino".pluralize(2) == "Dominoes"

I've added a series of new methods and removed reliance on NSRegularExpression. Comments and notes explain additions.

Added a method written in pure Swift - rangesOfString