This page provides information about the Light Path Expressions RE mode.
Overview
Light Path Expressions or LPEs are a very powerful and advanced tool for extracting specific lighting contribution events from the scene to a separate channel. This allows for very fine control of the image in compositing. For example, LPEs allow capturing only self-reflections, or the 1st bounce of GI, or the SSS that's only seen behind glass and similar for compositing control of only this aspect of the image.
The LPE syntax is based on regular expressions. Expressions describe a light path that starts from the camera and bounces around the scene until it reaches a light source.
The scene setup is similar to the Light Select workflow. As LPEs are a mode of the Light Select, the expression will capture contributions of the light that is added to the Light Select set. If the Light Select is empty, the expression will extract contributions from all scene lights. LPEs also allow extracting information from the environment, for example allow capturing the GI that comes from the environment only.