Chemformer's vocabulary is constructed by applying regular expression matching (we use the same regex as the Molecular Transformer ) to the canonical SMILES of the molecules in the ChEMBL 27 . There are 523 tokens in the vocabulary in total, including a large number of unused tokens which can be replaced with task-specific tokens, as required. Tokenisation and augmentation of SMILES was performed by extending the PySMILESUtils framework .