RegEx Match multiple times in string

You can try one of these:

(?<=<<)[^>]+(?=>>)
(?<=<<)\w+(?=>>)

However you will have to iterate the returned MatchCollection.

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

While Peter's answer is a good example of using lookarounds for left and right hand context checking, I'd like to also add a LINQ (lambda) way to access matches/groups and show the use of simple numeric capturing groups that come handy when you want to extract only a part of the pattern:

using System.Linq;
using System.Collections.Generic;
using System.Text.RegularExpressions;

// ...

var results = Regex.Matches(s, @"<<(.*?)>>", RegexOptions.Singleline)
            .Cast<Match>()
            .Select(x => x.Groups[1].Value);

Same approach with Peter's compiled regex where the whole match value is accessed via Match.Value:

var results = regex.Matches(s).Cast<Match>().Select(x => x.Value);

Note:

• <<(.*?)>> is a regex matching <<, then capturing any 0 or more chars as few as possible (due to the non-greedy *? quantifier) into Group 1 and then matching >>
• RegexOptions.Singleline makes . match newline (LF) chars, too (it does not match them by default)
• Cast<Match>() casts the match collection to a IEnumerable<Match> that you may further access using a lambda
• Select(x => x.Groups[1].Value) only returns the Group 1 value from the current x match object
• Note you may further create a list of array of obtained values by adding .ToList() or .ToArray() after Select.

In the demo C# code, string.Join(", ", results) generates a comma-separated string of the Group 1 values:

var strs = new List<string> { "this is a test for <<bob>> who like <<books>>",
                              "test 2 <<frank>> likes nothing",
                              "test 3 <<what>> <<on>> <<earth>> <<this>> <<is>> <<too>> <<much>>." };
foreach (var s in strs) 
{
    var results = Regex.Matches(s, @"<<(.*?)>>", RegexOptions.Singleline)
            .Cast<Match>()
            .Select(x => x.Groups[1].Value);
    Console.WriteLine(string.Join(", ", results));
}

Output:

bob, books
frank
what, on, earth, this, is, too, much