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The alpha numeric only filter removes any tokens that contain characters that are not ASCII letters (i.e. a to z and A to Z) or digits (i.e. 0 to 9). It is supported for all tokenizers besides the literal tokenizer. To enable, append alpha_num_only=true to the tokenizer’s arguments. 
CREATE INDEX search_idx ON mock_items
USING bm25 (id, (description::pdb.simple('alpha_num_only=true')))
WITH (key_field='id');
To demonstrate this token filter, let’s compare the output of the following two statements: 
SELECT
  'The café at 9pm!'::pdb.simple::text[],
  'The café at 9pm!'::pdb.simple('alpha_num_only=true')::text[];
Expected Response
       text        |     text
-------------------+--------------
 {the,café,at,9pm} | {the,at,9pm}
(1 row)