I am using StreamReader to read each line of a CSV file into a string. As I am processing each line, I need to strip out any commas that ONLY are surrounded by other numbers. For example, if the string was: "textfield1", "textfield2", "100.00", "1,070.00" I would need to only take the comma out of "1,070.00" from this entire string, resulting in: "textfield1", "textfield2", "100.00", "1070.00" Each string read from the CSV file can differ in number of fields, length, etc., so I need to use something (Regular Expressions maybe?) that looks at the entire string without hardcoding a location or blanket removal of all commas. Here is the approach I have been trying: StreamReader sr = new StreamReader(strInputFile); string nextLine = sr.ReadLine(); try { while ((nextLine = sr.ReadLine()) != null) { string rawtext = nextLine.Replace("[0-9]"+","+"[0-9]" , "[0-9]"+"[0-9]"); // ....rest of code } } This obviously doesn't work because I don't understand how to do this :) I am new at C# and inexperienced in Regex, so hopefully this is relatively simple.