How to style text that is enclosed in space characters

Via a paragraph style sheet, I need to apply character styles to text that is enclosed in space characters.

For example:

The cat <<thinspace>> sat <<thinspace>> on the <<enspace>>mat<<enspace>>

In the above, "Sat" would have one character style while "Mat" will have another.

What's the best way of doing this?

I know Indesign has GREP styles but that doesn't seem to work to text enclosed in space characters.

Also the only reason I am using space characters is because they are invisible when printed.

Is there any other method for wrapping text to mark it out as needing to be tagged with a style?

Thanks!