Getting all possible consecutive 4 digit numbers from a 10 digit number

Question

I am trying to make a regex to get all the possible consecutive 4 digit numbers from a 10 digit number. Like

num = \"2345678901\";
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Answer 1

Do you absolutely need to use Regex? The same operation can be achieved much more quickly using a simple loop.

private IEnumerable getnums(string num)
{
    for (int i = 0; i < num.Length - 3; i++)
    {
        yield return num.Substring(i, 4);
    }
}

private IEnumerable DoIt(string num)
{
    var res = Regex.Matches(num, @"(?=(\d{4}))")
                .Cast()
                .Select(p => p.Groups[1].Value)
                .ToList();
    return (IEnumerable)res;

}

On average the simple loop takes about half the time of the RegEx version.

static void Main(string[] args)
{

    var num = "2345678901";

    Stopwatch timer = new Stopwatch();

    timer.Start();
    foreach (var number in getnums(num))
    {
        // Yum yum numbers
    }
    timer.Stop();
    Console.WriteLine(timer.Elapsed.Ticks);

    timer.Reset();

    timer.Start();
    foreach (var number in DoIt(num))
    {
        // Yum yum numbers
    }
    timer.Stop();
    Console.WriteLine(timer.Elapsed.Ticks);
}