RegEx to find and replace a word

You could do this using regex as follows:

using System;
using System.Text.RegularExpressions;

class Program
{
   static void Main()
   {
      string text1 = "The some loathsome file";
      string text2 = "The some file as some.jpg sometimes";      
      string word = "some";
      string replace = "same";
      string regExp = @"\b" + word + @"\b(?!\.\w{1,4})";
      text1 = Regex.Replace(text1, regExp, replace); 
      Console.WriteLine(text1);
      text2 = Regex.Replace(text2, regExp, replace); 
      Console.WriteLine(text2);
      Console.ReadKey();
   }
}

The output is:

The same loathsome file
The same file as some.jpg sometimes

Notice that 'some' is only replaced by 'same' if it's a complete word.

So, 'loathsome' and 'sometimes' are not changed.
 
Notice also that the regex word boundary meta-character '\b' treats some.jpg as two words: 'some' and '.jpeg'. To avoid replacing 'some' here, we need to use a negative lookahead so it will only be matched if it's not immediately followed by a dot and between 1 and 4 'word' characters.