Trouble with Simple RegExp_Instr

Question

We have a need to examine a string to see if there are any alpha characters and I decided to learn something new and use some of the new RegExp functions. I hope someone can show me the error of my ways. I am using the class alpha but the only letter I find is "A" or "a".
DB and SQLPlus version 11.1.0.6.0 or 10.2.0.4.0.
07:54:55 sys@itsdb>
07:54:55 sys@itsdb> SELECT REGEXP_INSTR ('ABCDEFG',
07:54:55 2 '[:alpha:]',
07:54:55 3 1,
07:54:55 4 1,
07:54:55 5 0,
07:54:55 6 'i'
07:54:55 7 ) alpha_pos
07:54:55 8 FROM DUAL;
ALPHA_POS
1
1 row selected.
07:54:55 sys@itsdb>
07:54:55 sys@itsdb> SELECT REGEXP_INSTR ('123456ABCDEFG',
07:54:55 2 '[:alpha:]',
07:54:55 3 1,
07:54:55 4 1,
07:54:55 5 0,
07:54:55 6 'i'
07:54:55 7 ) alpha_pos
07:54:55 8 FROM DUAL;
ALPHA_POS
7
1 row selected.
07:54:55 sys@itsdb>
07:54:55 sys@itsdb> SELECT REGEXP_INSTR ('GFEDCBA123456',
07:54:55 2 '[:alpha:]',
07:54:55 3 1,
07:54:55 4 1,
07:54:55 5 0,
07:54:55 6 'i'
07:54:55 7 ) alpha_pos
07:54:55 8 FROM DUAL;
ALPHA_POS
7
1 row selected.
07:54:55 sys@itsdb>
07:54:55 sys@itsdb> SELECT REGEXP_INSTR ('BCDEFG123456',
07:54:55 2 '[:alpha:]',
07:54:55 3 1,
07:54:55 4 1,
07:54:55 5 0,
07:54:55 6 'i'
07:54:55 7 ) alpha_pos
07:54:55 8 FROM DUAL;
ALPHA_POS
0
1 row selected.

Answer 1

Very true, my bad, thanks Max ;)
Borrowing from your suggestion and moving to the other point Marc mentioned about obtaining the information on whether the 4th position in the string is a letter or not:
SQL> with t as (
  2  select 'ABCDEFG' col1 from dual union
  3  select 'abc4' col1 from dual union
  4  select '123456ABCDEFG' col1 from dual union
  5  select '123.56' col1 from dual union
  6  select 'ZZZ123456ABCDEFG' col1 from dual union
  7  select '11111' col1 from dual union
  8  select '123A56' col1 from dual union
  9  select 'GFEDCBA123456' col1 from dual)
10  -- end of sample data
11  select col1,
12         regexp_instr(col1, '[[:alpha:]]') ex1,
13         regexp_instr(col1, '^(.{3}[[:alpha:]])', 1, 1, 0, 'i') is_pos_4_letter
14    from t
15  /
COL1                    EX1 IS_POS_4_LETTER
11111                     0               0
123.56                    0               0
123456ABCDEFG             7               0
123A56                    4               1
ABCDEFG                   1               1
GFEDCBA123456             1               1
ZZZ123456ABCDEFG          1               0
abc4                      1               0
8 rows selected
SQL>