Single match

string pattern = ":(.*?):";
string lookup = "--:text in here:--";

// Instanciate your regex object and pass a pattern to it
Regex rgxLookup = new Regex(pattern, RegexOptions.Singleline, TimeSpan.FromSeconds(1));
// Get the match from your regex-object
Match mLookup = rgxLookup.Match(lookup);

// The group-index 0 always covers the full pattern.
// Matches inside parentheses will be accessed through the index 1 and above.
string found = mLookup.Groups[1].Value;