Using another table in BigQuery Regex - regex

Question

I would like to map a string column to a category based on a regular expression match.
Is it possible to use another bigquery table containing the regular expressions and corresponding category for this? This would make it easier for me to update only a table when adding new categories/updating the regex, instead of having to update all queries that would use this lookup.
Query:
CASE
-- Use the entries from another table here
WHEN REGEXP_MATCH(string_to_check, cat1regex) THEN cat1
WHEN REGEXP_MATCH(string_to_check, cat2regex) THEN cat2
etc.
END
Mapping table:
Regex category
pagex|pagey xy
pagez|page1 z1
It's also possible there is another simple way to do something similar that I'm not thinking of, answers pointing those out are welcome too.
Any help would be appreciated.

Answer 1

Below is for BigQuery Standard SQL
#standardSQL
SELECT
string_to_check,
MAX(IF(REGEXP_CONTAINS(string_to_check, reg), category, NULL)) AS category
FROM yourTable
CROSS JOIN mappingTable
GROUP BY string_to_check
You can test / play with it using below dummy date from your question
#standardSQL
WITH `mappingTable` AS (
SELECT r'pagex|pagey' AS reg, 'xy' AS category UNION ALL
SELECT r'pagez|page1', 'z1'
),
`yourTable` AS (
SELECT string_to_check
FROM UNNEST(["pagex.com", "pagez#example.org", "page.example.net"]) AS string_to_check
)
SELECT
string_to_check,
MAX(IF(REGEXP_CONTAINS(string_to_check, reg), category, NULL)) AS category
FROM yourTable
CROSS JOIN mappingTable
GROUP BY string_to_check