remove all characters aside from the decimal numbers immediately followed by the £ sign

var result = first / (1 / (decimal)second);

extension String{

    private static let decimalFormatter:NumberFormatter = {
        let formatter = NumberFormatter()
        formatter.allowsFloats = true
        return formatter
    }()

    private var decimalSeparator:String{
        return String.decimalFormatter.decimalSeparator ?? "."
    }

    func isValidDecimal(maximumFractionDigits:Int)->Bool{

        // Depends on you if you consider empty string as valid number
        guard self.isEmpty == false else {
            return true
        }

        // Check if valid decimal
        if let _ = String.decimalFormatter.number(from: self){

            // Get fraction digits part using separator
            let numberComponents = self.components(separatedBy: decimalSeparator)
            let fractionDigits = numberComponents.count == 2 ? numberComponents.last ?? "" : ""
            return fractionDigits.characters.count <= maximumFractionDigits
        }

        return false
    }

}

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

    // Get text
    let currentText = textField.text ?? ""
    let replacementText = (currentText as NSString).replacingCharacters(in: range, with: string)

    // Validate
    return replacementText.isValidDecimal(maximumFractionDigits: 2)

}

(\d\.\d)|[^a-zA-Z\d]

re = /(\d\.\d)|[^a-zA-Z\d]/
str = 'test1.3eb@j$2.xyz.'
subst = '\\1'

result = str.gsub(re, subst)

puts result