Join values only on a selected number of digits in Oracle SQL

I am looking to do a join from a column that has values with multiple digits (4 or 5 digits numbers) with another column also with digits (2,3,4,5 digits). The join should only be done on the 5 digit numbers.

Input         
Post_full     Post_new
23            11
2345          23           
23456         234          
34567         2345         
              23456        
Output
23456   (As this is the only 5 digit value in the input on both sides)

Answer

Use the length() function in the WHERE clause:

select t1.Post_full
from table1 t1 inner join table2 t2
on t1.Post_full = t2.Post_new
where length(t1.Post_full) = 5 and length(t2.Post_new) = 5