This is ugly but it works. You would be better suited created a .net function or something, but I wanted to see if I could do it strictly in VC for my bag of tricks:
{REGEX(Replace|^^^^^^^^^^^^^^^^^^^^|^.{{STRING(Length|TestString1)}}|TestString1)}
There are 20 (the width of the fixed-width field) '^' characters. From this, replace with your value from the start of the string, for the length of your value.