That will select (by a regex) every book which has a title starting with a number, is that what you want?
SELECT * FROM books WHERE title ~ '^[0-9]'
if you want integers which start with specific digits, you could use:
SELECT * FROM books WHERE CAST(price AS TEXT) LIKE '123%'
or use (if all your numbers have the same number of digits (a constraint would be useful then))
SELECT * FROM books WHERE price BETWEEN 123000 AND 123999;
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