StringToDouble.swift

extension String {
    /** If the string represents an double that fits into an Double, returns the corresponding double. This accepts strings that match the regular expression "[+-]?(?:\d*[\.,])?\d+" only. **/
    func toDouble() -> Double? {
        
        let pattern = "^[+-]?(?:\\d*[\\.,])?\\d+$"
        
        var error: NSError? = nil
        
        if let regex = NSRegularExpression(pattern: pattern, options: NSRegularExpressionOptions.DotMatchesLineSeparators, error: &error) {
            
            let numberOfMatches = regex.numberOfMatchesInString(self, options: nil, range: NSMakeRange(0, countElements(self)))
            
            if numberOfMatches != 1 {
                return nil
            }
            
            let dottedString = self.stringByReplacingOccurrencesOfString(",", withString: ".", options: NSStringCompareOptions.LiteralSearch, range: nil)
            
            return strtod(dottedString, nil)
        }
        
        return 0.0
    }
}