Setting a relevant expression match to false in a do…while

问题

I'm trying to write some very basic code, but I'm also challenging myself on regular expression. I've been able to muddle through the code up to a point, but where I'm really having a problem is that I'm trying to run a do...while loop while the expression is false. At this point in time I get absolutely no errors, but the do...while loop keeps running.

I'm attaching the relevant code below, here's hoping it helps.

Thank you in advance

if (tollResponse == "yes")
{
    Console.WriteLine("How much do you pay per trip?");
    string tollTax = Console.ReadLine();
    Match toll = Regex.Match (tollTax, @"[\d .]+");

    if (toll.Success)
    {
        Math.Round(Convert.ToDecimal(tollTax), 2);
        Console.WriteLine("Good lord that's high... well it's your money");
    }
    else
    {
        do
        {
            Console.WriteLine("Please enter a proper number");
            tollTax = Console.ReadLine();
        }
        while (toll.Success == false);
    }
}

回答1:

Simple coding bug...comments added to your code to explain issue

if (tollResponse == "yes")
{
    Console.WriteLine("How much do you pay per trip?");
    string tollTax = Console.ReadLine();
    //toll.Success gets set here.
    Match toll = Regex.Match(tollTax, @"[\d .]+");

    if (toll.Success)
    {
        //Not sure why you are doing this since you aren't using it in the given code
        Math.Round(Convert.ToDecimal(tollTax), 2);
        Console.WriteLine("Good lord that's high... well it's your money");
    }
    else
    {
        //This is an infinite loop because toll.Success is never set again.
        do
        {
            Console.WriteLine("Please enter a proper number");
            tollTax = Console.ReadLine();
        } while (toll.Success == false);
    }
}

---

What i think you want

if (tollResponse == "yes")
{
    Console.WriteLine("How much do you pay per trip?");

    //Loop over the Console.ReadLine() using the else statement and exit if it is right the first time
    do
    {
        string tollTax = Console.ReadLine();
        //toll.Success gets set here.
        Match toll = Regex.Match(tollTax, @"^[+-]?[0-9]{1,3}(?:,?[0-9]{3})*(?:\.[0-9]{2})?$");

        if (toll.Success)
        {
            Console.WriteLine("Good lord that's high... well it's your money");
            //will exit
        }
        else
        {
            Console.WriteLine("Please enter a proper number");
        } 
    } while (toll.Success == false);
}

Note: Removed 1 line of duplicate code as well, updated to use my recommended regex and removed Math.Round

---

Regex Tool

[\d .]+

Debuggex Demo

A more Valid Regex for currency

decimal optional (two decimal places)

^[+-]?[0-9]{1,3}(?:,?[0-9]{3})*(?:\.[0-9]{2})?$

Debuggex Demo

Explained:

number (decimal required)

^[+-]?[0-9]{1,3}(?:,?[0-9]{3})*\.[0-9]{2}$

Options: case insensitive

Assert position at the beginning of the string «^»
Match a single character present in the list below «[+-]?»
   Between zero and one times, as many times as possible, giving back as needed (greedy) «?»
   The character “+” «+»
   The character “-” «-»
Match a single character in the range between “0” and “9” «[0-9]{1,3}»
   Between one and 3 times, as many times as possible, giving back as needed (greedy) «{1,3}»
Match the regular expression below «(?:,?[0-9]{3})*»
   Between zero and unlimited times, as many times as possible, giving back as needed (greedy) «*»
   Match the character “,” literally «,?»
      Between zero and one times, as many times as possible, giving back as needed (greedy) «?»
   Match a single character in the range between “0” and “9” «[0-9]{3}»
      Exactly 3 times «{3}»
Match the character “.” literally «\.»
Match a single character in the range between “0” and “9” «[0-9]{2}»
   Exactly 2 times «{2}»
Assert position at the end of the string (or before the line break at the end of the string, if any) «$»

Will Match:

1,432.01
456.56
654,246.43
432
321,543
14239729
21379312.32

Will not Match

324,123.432
,,,312,.32
123,.23

taken from my answer here php - regex - how to extract a number with decimal (dot and comma) from a string (e.g. 1,120.01)?

回答2:

The problem is that your toll variable is not updated in your loop.

You would have to add this to your do block to make it work:

toll = Regex.Match (tollTax, @"[\d .]+");

来源：https://stackoverflow.com/questions/24690630/setting-a-relevant-expression-match-to-false-in-a-do-while