REM Count how many letter there are  

select regexp_count('ABC123', '[A-Z]'), /* This matches "A", "B" & "C" so returns 3 */ 
       regexp_count('A1B2C3', '[A-Z]')  /* This matches "A", "B" & "C" so returns 3 */ 
from   dual

REM Count how many times one letter is followed by one number 

select regexp_count('ABC123', '[A-Z][0-9]'), /* This matches "C1" so returns 1 */ 
       regexp_count('A1B2C3', '[A-Z][0-9]')  /* This matches "A1", "B2" & "C3" so returns 3 */ 
from   dual

REM Count how many times one letter is followed by one number at the start of a string 

select regexp_count('ABC123', '^[A-Z][0-9]') /* This has no match so returns 0 */, 
       regexp_count('A1B2C3', '^[A-Z][0-9]') /* This matches "A1" so returns 1 */ 
from   dual

REM Count how many times a letter is followed by exactly two numbers 

select regexp_count('ABC123', '[A-Z][0-9]{2}'), /* This matches "C12" so returns 1 */ 
       regexp_count('A1B2C3', '[A-Z][0-9]{2}') /* This has no match so returns 0 */ 
from   dual

REM Count how many times there are two occurrences of one letter followed by one number 

select regexp_count('ABC123', '([A-Z][0-9]){2}'), /* This has no match so returns 0 */ 
       regexp_count('A1B2C3', '([A-Z][0-9]){2}') /* This matches "A1B2" so returns 1 */ 
from   dual