regex - Regular expression with 6 characters in ASP.NET -

i need regular expression in asp.net accept 6 characters ,

1. at second position should accept underscore (_) and
2. at third position should accept maybe underscore (_) or hash (#) and
3. at fourth position should accept hash (#).

note: in regex user can enter: number, alphabets or star (*) @ position instead of above mentioned positions.

can 1 me out on this?

like:

1. a__cde
2. ab##cd
3. a_#cde
4. ******
5. abcdef
6. 123456

try following expression:

(?i)^(?=^(?:[a-z0-9*]*(?:#{1,2}|_{1,2})[a-z0-9*]*|[a-z0-9*]*)$).{6}$ 

will match:

• a__cde
• ab##cd
• ******
• abcdef
• 123456

won't match:

• a_#cde
• a_##aa
• a__#bc
• _##*