iOSエンジニアの正規表現入門 ref: http://qiita.com/shoheiyokoyama/items/5dc67fdc9e06a9dc5728

file0.swift

extension String {
    func isNumber() -> Bool {
        let pattern = "^[\\d]+$"
        guard let regex = try? NSRegularExpression(pattern: pattern) else { return false }
        let matches = regex.matches(in: self, range: NSRange(location: 0, length: length))
        return matches.count > 0
    }
}

let numberString = "0123"
print(numberString.isNumber()) // true

let alphabet = "abcd"
print(alphabet.isNumber()) // false