Try something like this:
string abc = ";abc: 123 ;234 ;345 ;def: 234 ;456 ;xyz: 123;";
Regex pattern = new Regex(";(?<val>[^;:]+?):", RegexOptions.ExplicitCapture);
IEnumerable<string> values = pattern.Matches(abc).Cast<Match>().Select(m => m.Groups["val"].Value);