BUG #5831: Splitting string into char array with string_to_array does not work

The following bug has been logged online:

Bug reference: 5831
Logged by: Splitting string into char array with string_to_array
Email address: valgog(at)gmail(dot)com
PostgreSQL version: 9.0.1
Operating system: Ubuntu/Linaro 4.4.4-14ubuntu5
Description: Splitting string into char array with string_to_array
does not work
Details:

Hi,

I cannot really remember now for sure, but the splitting string into array
using an empty string ('') worked for me on pre-9.0 versions to get array of
string chars. Now I have the following behavior:

postgres=# select string_to_array('ab', '' ),
regexp_split_to_array('ab', '' );
string_to_array │ regexp_split_to_array

─────────────────┼───────
───────────────
{ab} │ {a,b}
(1 row)

Why an empty string given as a separator to string_to_array is not splitting
the string at all?