PostgreSQL not using any index in regex search

Question

I have the following SQL statement to filter data with a regex search:

select * from others.table 
where vintage ~* '(17|18|19|20)[0-9]{2,}'

Upon some researching, I found that I need to create gin/gist index for better performance:

create index idx_vintage_gist on others.table using gist (vintage gist_trgm_ops);
create index idx_vintage_gin on others.table using gin (vintage gin_trgm_ops);
create index idx_vintage_varchar on others.table using btree (vintage varchar_pattern_ops);

Looking at the explain plan, it is not using any index but a seq scan:

Seq Scan on table t  (cost=0.00..45412.25 rows=1070800 width=91) (actual time=0.038..8518.830 rows=1075980 loops=1)
  Filter: (vintage ~* '(17|18|19|20)[0-9]{2,}'::text)
  Rows Removed by Filter: 25400
Planning Time: 0.481 ms
Execution Time: 8767.998 ms

There are total 1101380 rows in the table.

My question is why is it not using any index for the regex search?