What is the opposite of string interpolation?

Suppose you have a string:

$MY_PATH/test.txt

And suppose MY_PATH is defined as /foo/bar. After interpolation, the string now looks like this:

/foo/bar/test.txt

What do you call the opposite function? That is, what do you call the act of going from /foo/bar开发者_JAVA百科/test.txt to $MY_PATH/test.txt? Extrapolation?