Oracle substr Function usage

Oracle's use of the SUBSTR function
Gets a string that specifies the starting position and length of the string substr (string, start_position,[length] )
Such as:
SUBSTR ('This is a test', 6, 2) wouldreturn ' is'
SUBSTR ('This is a test', 6) wouldreturn 'Is a test'
SUBSTR ('techonthenet', -3, 3) wouldreturn 'Net'
SUBSTR ('techonthenet', -6, 3) wouldreturn ' the'

Select substr (' Thisisatest ', -4, 2) value from dual The result is te

Select substr (' Emros ', -3,1) value from dual The result is R

substr (' ABCDE ', -6) = Null 
substr (' ABCDE ', -5) = ' ABCDE '  
substr (' ABCDE ', -4) = ' bcde '  
substr (' ABCDE ', -2) = ' de '  
substr (' ABCDE ', -0) = ' ABCDE '

Oracle substr Function usage