Вот мое предложение для вас:
(?:test\s)(?<word>\b\S+\b)
Объяснение
(?:test\s) matches "test" with a trailing blank, but does not include it in the capture
(?<word>\b\s+\b) matches a word with any character except a whitspace. It will stored in a groupd called "word"
Вот небольшой пример того, как его использовать:
var regex = new Regex(@"(?:test\s)(?<word>\b\S+\b)");
var matchCollection = regex.Matches("test abcde dummy test fghij ");
foreach (Match match in matchCollection)
{
Debug.WriteLine(match.Groups["word"].Value);
}