c# generate random string based on pattern

Question

I'm trying to do a simple string generation based on pattern.
My idea was to use Regex to do simple replace. I've started with simple method:

private static string parseTemplate(string template)
{
    return Regex.Replace(template, @"(\[d)((:)?([\d]+)?)\]", RandomDigit());
}

private static string RandomDigit()
{
    Random r = new Random();
    return r.Next(0, 9).ToString();
}

What this does for now is replacing groups like [d], [d:3] with what supposed to be random digit.
Unfortunately every group is replaced with the same digit, for example if I put test [d][d][d:3] my method will return test 222.
I would like to get different digit in every place, like test 361.

Second problem I have is way to handle length:

right now I must specify [d] for every digit I want, but it would be easier to specify [d:3] and get the same output.

I know that there is a project called Fare, but I would like to do this without this library

For now I only search for [d], but is this method will work fine there won't be a problem to add other groups for example: [s] for special characters or any other type of patters.

Edit1

As it was suggested I changed Random to a static variable like so:

private static string parseTemplate(string template)
    {
        return Regex.Replace(template, @"(\[d)((:)?([\d]+)?)\]", RandomDigit());
    }

    private static Random r = new Random();

    private static string RandomDigit()
    {
        return r.Next(0, 9).ToString();
    }

Problem is that when I call my code like so:

Console.WriteLine(parseTemplate("test [d:2][d:][d]"));
Console.WriteLine(parseTemplate("test [d:2][d:][d]")); 

I get output like this

test 222
test 555

I would like output like this (for example):

test 265
test 962

I think that problem is with Regex.Replace which calls my RandomDigit only once.

Answer 1

For your first issue: When you call new Random() you are seeding with the same value every time you call the function - initialise a static Random member variable once then return r.Next(0,9).ToString();

Edit:

In answer to your comment, try using MatchEvaluator with a delegate, something like the following (untested):

static string RandomReplacer(Match m)
{
    var result = new StringBuilder();
    foreach (char c in m.ToString())
        result.Append(c == 'd' ? RandomDigit() : c);
    return result.ToString()
}

private static string parseTemplate(string template)
{
    return Regex.Replace(template, @"(\[d)((:)?([\d]+)?)\]", new MatchEvaluator(RandomReplacer));
}

You can then extend this approach to match [d:3] and parse it in your MatchEvaluator accordingly, solving your second issue.

Answer 2

Assumnig [d:3] means "three random digits", the following MatchEvaluator uses the length (read from group 4) to generate a random digit string:

static string ReplaceSingleMatch(Match m)
{
    int length;
    if (!int.TryParse(m.Groups[1].Value, out length))
        length = 1;
    char[] chars = new char[length];
    for (int i = 0; i < chars.Length; i++)
        chars[i] = RandomDigit()[0];
    return new string(chars);
}

You can then call this as follows:

private static string parseTemplate(string template)
{
    return Regex.Replace(template, @"\[d(?::(\d+))?\]", ReplaceSingleMatch);
}

You might want to then change RandomDigit to return a single char rather than a string, or to take an int and return multiple characters.

Answer 3

private static string GenerateMask(string mask)
{
    StringBuilder output = new StringBuilder();
    for (int i = 0; i < mask.Length; i++)
    {
        if (mask[i] == 'd' && mask[i - 1] != '\\')
        {
            int quantifier = 1;

            if (mask[i + 1] == ':')
                Int32.TryParse(mask[i + 2].ToString(), out quantifier);

            output.Append(GetRandomDigit(quantifier));
            i += 2;
        }
        else
        {
            if(mask[i] != '\\')
            output.Append(mask[i]);
        }
    }

    return output.ToString();
}

private static string GetRandomDigit(int length)
{
    Random random = new Random();
    StringBuilder output = new StringBuilder();
    while (output.Length != length)
        output.Append(random.Next(0, 9));
    return output.ToString();
}

There's a custom algorithm I just put together for fun mostly and here's the implementation:

Console.WriteLine(GenerateMask(@"Hey Da\d, meet my new girlfrien\d she's d:2"));
//Output: Hey Dad, meet my new girlfriend she's 44