Indexing fragments of a column's value ?

I'm asking about the possibility of indexing portions of a column's value
where the column has a static field format. Example, a char(8) which
contains all hex values (basically a hex number that's always 8 chars wide,
leading zeros if needed). Someone might want to select all recs where the
first 2 digits are 'ff' or maybe the last 4 hex digits match regexp_match
'00[cdef]{2}' or maybe a match of the entire string... "0dd63a87".

If I know the placement and width of the fields that need to be indexed,
can indices be defined to facilitate queries ? Example...
- match all 8 chars
- match the 3rd and 4th chars
- match the last 4 chars

I suppose I could fragment the thing into multiple columns for the purposes
of a search. So add a column called "last4" as a char(4) that matches the
last 4 chars of that column. Then index that. etc... But inquiring to see
if there is something more elegant.

Thanks !