cleanRegex.swift

import Foundation

func cleanRegex(pattern: String) -> String {
  let res = pattern.characters.enumerate().reduce(([], [])) { (acc, enumerator) -> ([Int], [Int]) in
    
    if enumerator.element == "{" {
      return (acc.0 + [enumerator.index], acc.1)
    }
    
    if enumerator.element == "}" {
      
      if let lastOpen = acc.0.last {
        let range = NSRange(location: lastOpen, length: enumerator.index - lastOpen + 1)
        
        if isQuantifier(pattern, range: range) {
          return (Array(acc.0.dropLast()), acc.1)
        } else {
          return (acc.0, acc.1 + [enumerator.index])
        }
      } else {
        return (acc.0, acc.1 + [enumerator.index])
      }
    }

    return acc
  }
  
  let allBrackets = res.0 + res.1
  
  let characters =
    pattern.characters.enumerate().reduce([Character]()) { (acc, enumerator) -> [Character] in
    if allBrackets.contains(enumerator.index) {
      return acc + ["\\", enumerator.element]
    }
    
    return acc + [enumerator.element]
  }
  
  return String.init(characters)
}

func isQuantifier(pattern: String, range: NSRange) -> Bool {
  guard
    let reg = try? NSRegularExpression.init(pattern: "\\{\\d{1,},?\\d*\\}",
                                            options: .DotMatchesLineSeparators) else {
                                              return false
  }
  
  return reg.firstMatchInString(pattern, options: .Anchored, range: range) != nil
}

let pattern = "{This is Kinda {1}Cool. It might {{}} ge{t} kinda }}{{{complicated}}"
print(cleanRegex(pattern))