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The regex_pattern tokenizer tokenizes text using a regular expression. The regular expression can be specified with the pattern parameter. For instance, the following tokenizer creates tokens only for words starting with the letter h: 
CREATE INDEX search_idx ON mock_items
USING bm25 (id, (description::pdb.regex_pattern('(?i)\bh\w*')))
WITH (key_field='id');
The regex tokenizer uses the Rust regex crate, which supports all regex constructs with the following exceptions:
  1. Lazy quantifiers such as +?
  2. Word boundaries such as \b
To get a feel for this tokenizer, run the following command and replace the text with your own: 
SELECT 'Hello world!'::pdb.regex_pattern('(?i)\bh\w*')::text[];
Expected Response
  text
---------
 {hello}
(1 row)