A common first step in chemical registration systems is to standardize molecules -removing fragments, transforming functional groups into a standard form, etc -and then to check each molecule against a set of filters to ensure that they are compliant with the rules of the registration system. These filters may include things like flagging molecules that are badly drawn, include certain undesired elements or functional groups, or that have unspecified or incorrectly specified stereochemistry. The standardization rules and filters that are applied are generally specific to a particular company or institution. In order to support this important part of registration, lwreg provides a flexible and configurable interface for standardizing molecules before they are registered and for filtering out molecules, which should not be added.