RegEx Match multiple times in string

Use a positive look ahead and look behind assertion to match the angle brackets, use .*? to match the shortest possible sequence of characters between those brackets. Find all values by iterating the MatchCollection returned by the Matches() method.

Regex regex = new Regex("(?<=<<).*?(?=>>)");

foreach (Match match in regex.Matches(
    "this is a test for <<bob>> who like <<books>>"))
{
    Console.WriteLine(match.Value);
}

LiveDemo in DotNetFiddle