I checked your CAPX. I noticed the number you want to format has a floating point. My Regex assume you don't. The "FormatCurrency" Regex assume there is a floating point. So you might have to combine both in a function and use the appropriate Regex. Be sure to pass the value as a string to the regex, otherwise it might behave strangely. You can do that by appending an empty string to the variable (implicit conversion), or using the str() function (explicit conversion).
Here's the modified CAPX.
tomhaiger You might want to read a bit on "regular expressions" in general, especially the Javascript variant, which lacks the lookbehind functionnalities. It's quite a powerful feature for matching and replacing strings.