string s = "(L11+L12+L22)*0.25+L18";
foreach (System.Text.RegularExpressions.Match m in System.Text.RegularExpressions.Regex.Matches(s, @"L\d+"))
{
    string value = m.Value;
}

------解决方案--------------------

//using System.Text.RegularExpressions;

foreach (Match m in Regex.Matches("(L11+L12+L22)*0.25+L18", @"L\d+"))
{
   Console.WriteLine(m.Value);
}