using System;
using System.Text;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var s = "Hi is this available 18dec to 21st dec 2nd dec 1st jan dec12th";
		var res = Regex.Replace(s, @"(\p{L})?(\d+)(st|[nr]d|th|(\p{L}+))", repl);
		Console.WriteLine(res);
	}
	public static string repl(Match m) 
	{
		var res = new StringBuilder();
		res.Append(m.Groups[1].Value);
		if (m.Groups[1].Success)
			res.Append(" "); 
		res.Append(m.Groups[2].Value);
		if (m.Groups[4].Success)
			res.Append(" ");
		res.Append(m.Groups[3]);
		return res.ToString();
	}
}

dXNpbmcgU3lzdGVtOwp1c2luZyBTeXN0ZW0uVGV4dDsKdXNpbmcgU3lzdGVtLkNvbGxlY3Rpb25zLkdlbmVyaWM7CnVzaW5nIFN5c3RlbS5JTzsKdXNpbmcgU3lzdGVtLkxpbnE7CnVzaW5nIFN5c3RlbS5UZXh0LlJlZ3VsYXJFeHByZXNzaW9uczsKCnB1YmxpYyBjbGFzcyBUZXN0CnsKCXB1YmxpYyBzdGF0aWMgdm9pZCBNYWluKCkKCXsKCQl2YXIgcyA9ICJIaSBpcyB0aGlzIGF2YWlsYWJsZSAxOGRlYyB0byAyMXN0IGRlYyAybmQgZGVjIDFzdCBqYW4gZGVjMTJ0aCI7CgkJdmFyIHJlcyA9IFJlZ2V4LlJlcGxhY2UocywgQCIoXHB7TH0pPyhcZCspKHN0fFtucl1kfHRofChccHtMfSspKSIsIHJlcGwpOwoJCUNvbnNvbGUuV3JpdGVMaW5lKHJlcyk7Cgl9CglwdWJsaWMgc3RhdGljIHN0cmluZyByZXBsKE1hdGNoIG0pIAoJewoJCXZhciByZXMgPSBuZXcgU3RyaW5nQnVpbGRlcigpOwoJCXJlcy5BcHBlbmQobS5Hcm91cHNbMV0uVmFsdWUpOwoJCWlmIChtLkdyb3Vwc1sxXS5TdWNjZXNzKQoJCQlyZXMuQXBwZW5kKCIgIik7IAoJCXJlcy5BcHBlbmQobS5Hcm91cHNbMl0uVmFsdWUpOwoJCWlmIChtLkdyb3Vwc1s0XS5TdWNjZXNzKQoJCQlyZXMuQXBwZW5kKCIgIik7CgkJcmVzLkFwcGVuZChtLkdyb3Vwc1szXSk7CgkJcmV0dXJuIHJlcy5Ub1N0cmluZygpOwoJfQp9