如果我们可以假设数字（门牌号，公寓号等）必须从数字1..9开始，并且可以以英语小写字母a..z结束，那么您可以尝试匹配 * 正则表达式 * 模式：

(?<number>[1-9][0-9]*[a-z]*)(?:\s+\k<number>)+

模式解释

(?<number>[1-9][0-9]*[a-z]*) - a group "number" which
                               starts from digit in 1..9 range
                               can followed by digits in 0..9 range
                               finally, can followed by letters in a..z range
          (?:\s+\k<number>)+ - the very same match "number" repeated 
                               one or more times separated by white spaces

代码：

using System.Linq;
using System.Text.RegularExpression;

...

string text = "Randomstreet 33a 33a or Randomstreet 45 45 apartment 14 14 14";

var result = Regex
  .Matches(text, @"(?<number>[1-9][0-9]*[a-z]*)(?:\s+\k<number>)+")
  .Select(match => match.Groups["number"].Value)
  .ToArray();

Console.Write(string.Join(Environment.NewLine, result));

输出：

33a
45
14