Hello Tom,

I am learning oracle regular expressions. I am using Oracle 12c.
I understand that a bracket expression containing a series of characters returns the first character in the input string.
Thus, select regexp_substr('123 Oracle', '[abc]') from dual
returns the chracter 'a'.
What if bracket expressions are nested?

select regexp_substr('123 Oracle', '[[abc]]') from dual

returns NULL indicating no match. But Why?

How does oracle interpret this regular expression?
Within the outer [] brackets, I expect the inner [] brackets to be treated as literals.
So the output should be the first character among the list [ a b c ] to be matched.
Once again regexp_substr must return 'a'. Why is NULL returned instead?

A variation on this:
select regexp_substr('[]123 Oracle', '[[abc]]') from dual
returns '[]'. And I thought that a bracketed expression would always return a single character.

Any suggestions?

Thanks
Geetha

select regexp_substr('123 Orac]le', '[[abc]]') r1,
       regexp_substr('123 Oracle', '[]abc[]') r2,
       regexp_substr('123 Oracle', '(\[|\]|[abc])') r3,
       regexp_substr('123[ Oracle', '[]abc[]') r4,
       regexp_substr('123[ Oracle', '(\[|\]|[abc])') r5
from   dual;

R1  R2  R3  R4  R5  
c]  a   a   [   [