If you mean "if the word 'Hundred" follows Thousand or ..." as in your example, then try this:
(?<=(Thousand|Hundred|Lakh)\s+)Hundred
To remove the word:
public static Regex remove = new Regex("(?<=(Thousand|Hundred|Lakh)\\s+)Hundred", RegexOptions.Compiled);
...
string result = remove.Replace(InputText,"");
If you want to play with regexes, get a copy of
Expresso[
^] - it's free, and it examines and generates Regular expressions.