Swift 3 String Extensions

String+Currency.swift

import Foundation

extension String
{
	/**
	* Return formated currency string by locale with or not round
	* @see https://developer.apple.com/documentation/foundation/numberformatter, https://developer.apple.com/documentation/foundation/numberformatter.style
	* @return String if problem on format return string ""
	*/
	func toCurrency(localeItendifier: String = "pt_BR", withRound: Bool = false) -> String{
		let numberFormatter: NumberFormatter = {
			let formater = NumberFormatter()
			formater.numberStyle = .currency
			formater.locale = Locale(identifier: localeItendifier)
			formater.alwaysShowsDecimalSeparator = true
			formater.minimumFractionDigits = 2
			formater.maximumFractionDigits = 2
			return formater
		}()
		let valToFloat = Float(self)!
		var val: NSNumber
		if(withRound){
			val = NSNumber(value: round(valToFloat))
		} else {
			val = NSNumber(value: valToFloat)
		}
		if let formmatedVal = numberFormatter.string(from: val){
			return formmatedVal
		}
		return ""
	}
	
	/**
	* Shortcut replacingOccurrences
	* @return String - string replaced
	*/
	func replace(this: String, to: String) -> String
	{
		return self.replacingOccurrences(of: this, with: to)
	}
	/**
	* Remove all another char did not match with characterset decimalDigits
	* @see https://developer.apple.com/documentation/foundation/characterset
	* @return String - String decimals
	*/
	func getDecimals() -> String
	{
		return components(separatedBy: CharacterSet.decimalDigits.inverted).joined()
	}
	/**
	* Verify if char is a utf8 backspace, util for testin keyboard keypress
	* @see https://developer.apple.com/documentation/swift/string/1641523-init
	* @see https://developer.apple.com/documentation/kernel/1579329-strcmp
	* @return Bool - if a backspace char return true else false
	*/
	func isBackSpace() -> Bool
	{
		let char = self.cString(using: String.Encoding.utf8)!
		let isBackSpace = strcmp(char, "\\b")
		if(isBackSpace == -92){ return true }
		return false
	}
}

Usage
var compras = "R$1000.99"
var comprasnumero = "12300.92"

print(comprasnumero.toCurrency())
print(comprasnumero.toCurrency(localeItendifier: "pt_BR", withRound: true))

print(compras.replace(this: "R$", to: ""))
print(compras.getDecimals())
print(compras.isBackSpace())