GREP is a symbolic system, and it is easy to lose track of certain formal aspects. We’re not helped by GREP’s interface: entering complex expressions can be a challenge in such a confined space, and the miniscule type in the “Find what” and “Change to” fields doesn’t help either. Nor are we warned about errors in an expression: InDesign accepts anything. Recalcitrant expressions can be frustrating, but there are a number of things to look out for when troubleshooting expressions.