In this work, we present PoreMatMod.jl, a new, free, open-source, tested, and documented software package, written in the Julia programming language [ ], to modify crystal structure models of porous materials. In contrast to prior structure generation algorithms that mimic de novo synthesis, PoreMatMod.jl produces hypothetical structures by modifying existing crystal structure models, mimicking PSM or BBR. PoreMatMod.jl functions, effectively, as a find-and-replace algorithm on labeled (by the chemical species) graph and point cloud representations of porous crystals. The user specifies a parent crystal structure, a query molecular fragment/moiety, and a replacement molecular fragment/moiety. PoreMatMod.jl then ( ) applies Ullman's algorithm [ ] to search for all subgraphs of the parent graph that are isomorphic to the query graph [ ], ( ) uses orthogonal Procrustes [ ] to align the replacement fragment on the matching substructure of the parent structure, then ( ) removes the query fragment and installs the replacement fragment in an optimal geometry, giving the child structure. When multiple subgraphs of the parent are isomorphic to the query, PoreMatMod.jl grants precise control over the distribution of the replacements. We demonstrate that PoreMatMod.jl is useful for: tuning the chemistry of existing crystal structure models of porous materials (e.g., curated databases of experimentally-reported structures in Refs. ) to generate a library of hypothetical materials for computational screening (e.g., ) generating heterogeneous, multi-linker MOFs with precise control of functional group placement (e.g., [ ]) repairing artifacts in crystal structures determined from X-ray diffraction (XRD) patterns, such as missing hydrogen atoms, disorder, and the presence of solvents (e.g., [ ]) introducing missing-linker and missing-SBU defects into MOFs to enable computational studies on the influence of such defects on properties (e.g., ) searching for subgraphs in libraries of crystal structure models to e.g., filter structures or characterize chemical diversity (e.g., [ ]).