Can an expression parse a text file or the expression of another property in After Effects? I am going to have tons of layers use the same expression, but I need to be able to edit the expression later without having to edit it for every layer. I also can't simply have each layer's property point to the original, because the expression needs to be evaluated for each layer relative to itself.

Is there a way I can have each layer expression simply point to and parse a text file? I could edit the text file so all layers expressions update.
Or, if somehow I could put the expression on a null object and have each layer parse that expression for itself.
Thanks for any help!

My workaround solution is to make a null object with a slider control for every single value that I may edit later, updating all layers, but this doesn't allow me to change the non-numerical information in the expression, such as adding parenthesis or other functions.
Yes, since expressions are basically JavaScript, you can load and evaluate external source files. Unfortunately the location needs to be hard-coded, since (AFAIK) expressions do not have access to the path of your AE project, so it's rather brittle. For instance:

try {
    myPath = "~/Desktop/source.js";
    $.evalFile(myPath);
} catch(err) {
    "MISSING";
}


Further explanation of how similar code works can be seen on the ProVideoCoalition site.

One further caveat is that the script is loaded and evaluated again for every frame that gets rendered. You might want to test how loading the script affects your render times.