camel.swift

public extension String {
    public func rangeFromNSRange(aRange: NSRange) -> Range<String.Index> {
        let s = advance(self.startIndex, aRange.location)
        let e = advance(self.startIndex, aRange.location + aRange.length)
        return s..<e
    }
    public var ns : NSString {return self as NSString}
    public subscript (aRange: NSRange) -> String? {
        get {return self.substringWithRange(self.rangeFromNSRange(aRange))}
    }
    
    public var cdr: String {return isEmpty ? "" : String(characters.dropFirst())}

    public func toSnake() throws -> String {
        var s = self
        let regex = try NSRegularExpression(pattern: "[:upper:]+", options: [])
        let matches = regex.matchesInString(s, options:[], range:NSMakeRange(0, s.ns.length)).reverse()
        for match in matches {
            let matchRange = s.rangeFromNSRange(match.range)
            let lc = s[match.range]!.lowercaseString
            s.replaceRange(matchRange, with: "_" + lc)
        }
        if s.hasPrefix("_") {s = String(s.characters.dropFirst())}
        return s
    }
    
    public func toScreamingSnake() throws -> String {
        return try self.toSnake().uppercaseString
    }
    
    public func toCamel() throws -> String {
        var s = self
        let regex = try NSRegularExpression(pattern: "_.", options: [])
        let matches = regex.matchesInString(s, options:[], range:NSMakeRange(0, s.ns.length)).reverse()
        for match in matches {
            let matchRange = s.rangeFromNSRange(match.range)
            let uc = s[match.range]!.uppercaseString.cdr
            s.replaceRange(matchRange, with: uc)
        }
        if s.hasPrefix("_") {s = s.cdr}
        return s
    }
}

let s = "helloWorld"
try s.toSnake()
try s.toSnake().toCamel()