Swift number validator

NumberValidator.swift

let valid = ["3.14159265", "123.12", "2", "56754", "92929292929292.12", "0.21", "3.1","3","-3","0","0.0","1.0","0.1", ".12", "-12", "-34.6"]
let invalid = ["e666.76","a"," ","-"," -1","--1", "2..3", "2-", "2...3", "2.4.3", "5-6-7"]

func isValidNumber(numberString: String) -> Bool {
    let numberFormatter = NumberFormatter()
//    print(numberFormatter.decimalSeparator)
    let decimalSeparator = numberFormatter.decimalSeparator ?? "."
    let regularExpression = "[+-]?((\\d+(\\\(decimalSeparator)\\d*)?)|(\\\(decimalSeparator)\\d+))"
//    print(regularExpression)

    guard let regex = try? NSRegularExpression(pattern: regularExpression, options: .caseInsensitive) else {
        fatalError("fail regex \(numberString)")
    }

    guard let firstMatch = regex.firstMatch(in: numberString, options: .init(rawValue: 0), range: NSMakeRange(0, numberString.characters.count)) else {
        return false
    }
//    print("first match: \(firstMatch.range)")

    let startIndex = numberString.index(numberString.startIndex, offsetBy: firstMatch.range.location)
    let endIndex = numberString.index(numberString.startIndex, offsetBy: firstMatch.range.location + firstMatch.range.length - 1)

    let matchString = numberString[startIndex...endIndex]
//    print("match string: \(matchString)")
    
    return numberString == matchString
}


for testString in valid {
    let isValid = isValidNumber(numberString: testString)
    if !isValid {
        print("error, should be valid: \(testString)")
    }
    print("is valid: \(isValid)")
}

for testString in invalid {
    let isValid = isValidNumber(numberString: testString)
    if isValid {
        print("error, should be invalid: \(testString)")
    }
    print("is invalid: \(!isValid)")
}