Four special symbols were added into the token vocabulary, which are "<bos>", "<eos>", "<mask>" and "<pad>", meaning respectively, the start token of a reaction SMILES string, end token of a reaction SMILES string, the mask token and placeholder of void tokens. Using all Reaxys ® and USPTO reactions as semantic input, the BPE method tokenised the reaction SMILES expression into 2,863 tokens including the four special tokens.