using System;
using System.Collections;
using System.Text.RegularExpressions;
public class Test
{
    public static void Main()
    {   
        string txt = "department_name:womens AND item_type_keyword:base-layer-underwear";
        var reg = new Regex(@"(?:department_name|item_type_keyword):([\w-]+)", RegexOptions.IgnoreCase);
        var ms = reg.Matches(txt);
        ArrayList results = new ArrayList();
        foreach (Match match in ms)
        {
            results.Add(match.Groups[0].Value);
            results.Add(match.Groups[1].Value);
        }
        // results is your final array containing all results
        foreach (string elem in results)
    	{
    		Console.WriteLine(elem);
    	}
    }
}

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