多個匹配

using System.Text.RegularExpressions;

List<string> found = new List<string>();
string pattern = ":(.*?):";
string lookup = "--:text in here:--:another one:-:third one:---!123:fourth:";

// Instanciate your regex object and pass a pattern to it
Regex rgxLookup = new Regex(pattern, RegexOptions.Singleline, TimeSpan.FromSeconds(1));
MatchCollection mLookup = rgxLookup.Matches(lookup);

foreach(Match match in mLookup)
{
    found.Add(match.Groups[1].Value);
}

結果：

found = new List<string>() { "text in here", "another one", "third one", "fourth" }