I am using a set of SQL LIKE conditions to go through the alphabet and list all items beginning with the appropriate letter, e.g. to get all books where the title starts with the letter "A":
SELECT * FROM books WHERE title ILIKE "A%"
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;