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As stated above, because the physical description field of the instance record is a text-entry field that has no enforced data entry format, the parsing statement will not yield highly accurate results. It works in cases where a page count number is followed by "p." or "pages" or "leaves" or "l." and when the first number in the description field (reading from left to right) is the actual number of pages or leaves, and not some other value. 

WITH parameters AS 
    (SELECT 
    'law,ref' AS location_code_filter ---- enter a location code between the quote marks; below
--if left blank, the query will get all locations except for 'serv,remo'

WITH parameters AS 
    (SELECT 
    'law,ref' AS location_code_filter 
    )

SELECT distinct
    instance.id AS instance_id,
    instance.jsonb#>>'{hrid}' AS instance_hrid,
    instance.jsonb#>>'{title}' AS title,
    location__t.name AS location_name,
    holdings_record__t.call_number,
    phys.jsonb#>>'{}' AS field_300,
    CASE
        WHEN substring (phys.jsonb#>>'{}','\d{1,4}') IS NOT NULL
        AND (phys.jsonb#>>'{}' LIKE '%page%' OR phys.jsonb#>>'{}' LIKE '%p.%' OR phys.jsonb#>>'{}' LIKE '%leaves%' OR phys.jsonb#>>'{}' LIKE '% l.%')
        THEN substring (phys.jsonb#>>'{}','\d{1,4}')::INT
        ELSE NULL
    END AS page_count_estimate,
    phys.ordinality AS record_sequence_ordinality

FROM folio_inventory.instance
    LEFT JOIN LATERAL jsonb_array_elements (jsonb_extract_path (instance.jsonb,'physicalDescriptions')) WITH ordinality AS phys (jsonb)
    ON true
    
    LEFT JOIN folio_inventory.holdings_record__t 
    ON instance.id = holdings_record__t.instance_id 
    
    LEFT JOIN folio_inventory.location__t 
    ON holdings_record__t.permanent_location_id = location__t.id 

WHERE 
    CASE WHEN (SELECT location_code_filter FROM parameters) =''
    THEN location__t.code != 'serv,remo' 
    ELSE location__t.code = (SELECT location_code_filter FROM parameters) 
    END 
;

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