Question: How can I find all indexes of a pattern in a string using c#? 

For example I want to find all `##` pattern indexes in a string like this `45##78$$#56$$JK01UU`

Answer: 
You can get all the indices of a pattern in a string by using a regex search like this.

```
string input = "45##78$$#56$$JK01UU", pattern = Regex.Escape("##");
Regex rx = new Regex(pattern);
var indices = new List<int>();
var matches = rx.Matches(s);
for (int i=0 ; i<matches.Length ; i++)
{
    indices.Add(matches[i].Index);
}

```
Another one that tries to be efficient:

```
public IEnumerable<int> FindPatternIndexes(string input, string search)
{
    var sb = new StringBuilder(input);

    for (var i = 0; search.Length <= sb.Length; i++)
    {
        if (sb.ToString().StartsWith(search)) yield return i;
        sb.Remove(0,1);
    }
}

```