Common Swift String Extensions

import Foundation

extension String
{
    var length: Int {
        get {
            return countElements(self)
        }
    }
    
    func contains(s: String) -> Bool
    {
        return self.rangeOfString(s) ? true : false
    }
    
    func replace(target: String, withString: String) -> String
    {
        return self.stringByReplacingOccurrencesOfString(target, withString: withString, options: NSStringCompareOptions.LiteralSearch, range: nil)
    }
    
    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
    }
    
    func isMatch(regex: String, options: NSRegularExpressionOptions) -> Bool
    {
        var error: NSError?
        var exp = NSRegularExpression(pattern: regex, options: options, error: &error)
        
        if let error = error {
            println(error.description)
        }
        var matchCount = exp.numberOfMatchesInString(self, options: nil, range: NSMakeRange(0, self.length))
        return matchCount > 0
    }
    
    func getMatches(regex: String, options: NSRegularExpressionOptions) -> [NSTextCheckingResult]
    {
        var error: NSError?
        var exp = NSRegularExpression(pattern: regex, options: options, error: &error)
        
        if let error = error {
            println(error.description)
        }
        var matches = exp.matchesInString(self, options: nil, range: NSMakeRange(0, self.length))
        return matches as [NSTextCheckingResult]
    }
    
    private var vowels: [String]
    {
      get
      {
        return ["a", "e", "i", "o", "u"]
      }
    }
    
    private var consonants: [String]
    {
      get
      {
        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)
        }
    }
}

Usage

"Awesome".contains("me") == true
"Awesome".contains("Aw") == true
"Awesome".contains("so") == true
"Awesome".contains("Dude") == false

"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: NSRegularExpressionOptions.CaseInsensitive) == true
"email-test.com".isMatch(emailRegex, options: NSRegularExpressionOptions.CaseInsensitive) == false

var testText = "[email protected], [email protected], [email protected]"
var matches = testText.getMatches(emailRegex, options: NSRegularExpressionOptions.CaseInsensitive)
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"

Totally disagree with @dstd: the word pluralize() indicates that it's an English thing. Otherwise it would be 复数() or pluralizar(), 不过当然啦，正常的中文里也没复数这种东西，孩儿们。

advance is not use in switf 1.x
use advanceby now

func trim() -> String
{
return self.trimmingCharacters(in: .whitespaces)
}

Cool! Just contains have been replaced with following:
string.containsString("abc")

In Swift 3, subscript method should be change to following code:

extension String {
    subscript(r: Range<Int>) -> String {
        get {
            let startIndex = self.index(self.startIndex, offsetBy: r.lowerBound)
            let endIndex = self.index(self.startIndex, offsetBy: r.upperBound)

            return self[startIndex..<endIndex]
        }
    }

    subscript(r: ClosedRange<Int>) -> String {
        get {
            let startIndex = self.index(self.startIndex, offsetBy: r.lowerBound)
            let endIndex = self.index(self.startIndex, offsetBy: r.upperBound)

            return self[startIndex...endIndex]
        }
    }
}

The String API of swift is way too much ugly and uncomfortable... T_T

Substring does not work anymore!?