BUG #6743: BETWEEN operator does not work for char(1)

The following bug has been logged on the website:

Bug reference: 6743
Logged by: Spătărel Dan
Email address: spatarel1(at)yahoo(dot)com
PostgreSQL version: 9.1.4
Operating system: Windows Vista SP2
Description:

I use "UTF8" charset and "Romania, Romanian" locale.

I came across this as I wanted to test if a symbol was a letter:

SELECT 'a' BETWEEN 'a' AND 'z'; -- true
SELECT 'z' BETWEEN 'a' AND 'z'; -- true
SELECT 'A' BETWEEN 'a' AND 'z'; -- true
SELECT 'Z' BETWEEN 'a' AND 'z'; -- false (!)
SELECT 'a' BETWEEN 'A' AND 'Z'; -- false (!)
SELECT 'z' BETWEEN 'A' AND 'Z'; -- true
SELECT 'A' BETWEEN 'A' AND 'Z'; -- true
SELECT 'Z' BETWEEN 'A' AND 'Z'; -- true

It seems that the intent is for the comparison to be case-insensitive, but
in some limit-cases it fails.

Please let me know if this turns out to be a real bug on not.