select * 
  from emp
 where regexp_like( ename, nvl( :P6_REGEXP, ename ), 'ix' ) 

Start with S: ^s

End with R:  r$

Start with S and end with H:  ^s.*h$

Start with S or end with R:  ^s|r$

All 4 letter names:  ^....$

Contains B, C, D or K:  [b-d,k]

All names with double letters:  (.)\1