Process string in C#

Split the string into words. If the words are always separated by single spaces, you can use String.Split[^]; otherwise, you might need to use a regular expression:

string[] words = System.Text.RegularExpressions.Regex.Matches(input, @"\w+")
    .Cast<System.Text.RegularExpressions.Match>()
    .Select(m => m.Value)
    .ToArray();

Regular Expression Language - Quick Reference | Microsoft Docs[^]

Then you need to generate all combinations of words. Eric Lippert has a nice series of blog posts explaining how to do that:
Producing combinations, part one | Fabulous adventures in coding[^]

Or you can look at the implementations on Rosetta Code:
Combinations - Rosetta Code[^]

A simple version might look something like this:

static IEnumerable<IReadOnlyList<T>> AllCombinations<T>(IReadOnlyList<T> input)
{
    for (int m = input.Count; m >= 1; m--)
    {
        var result = new T[m];
        var stack = new Stack<int>();
        stack.Push(0);
        
        while (stack.Count > 0)
        {
            int index = stack.Count - 1;
            int value = stack.Pop();
            while (value < input.Count)
            {
                result[index] = input[value];
                index++;
                value++;
                
                stack.Push(value);
                
                if (index == m)
                {
                    yield return result;
                    break;
                }
            }
        }
    }
}

You then combine the two:

IEnumerable<string> parts = AllCombinations(words).Select(c => string.Join(" AND ", c));
string result = "(" + string.Join(") OR (", parts) + ")";

You have to

• Split the string into its words (an array of strings)
• make a loop, with n=1,..,len where len is the length of the words array
• at iteration n, find the combinations of n items of the words array
• iterate over the found combinations printing them properly formatted

Findindig the combinations could be tricky. This page gives great help: c# - How to use LINQ to find all combinations of n items from a set of numbers? - Stack Overflow[^].