^[0-9,_,%]{1,20}$

^[0-9,_,%]{1,20}$

select 170 as c from dual where not regexp_like('12345810_%','[^[:digit:]_,%]|.{21,}')
union all
select 171 from dual where not regexp_like('%','[^[:digit:]_,%]|.{21,}')
union all
select 42 from dual where not regexp_like('!','[^[:digit:]_,%]|.{21,}')
union all
select 43 from dual where not regexp_like('12345810_%!','[^[:digit:]_,%]|.{21,}')
union all
select 44 from dual where not regexp_like('123458101234567890123456789012345678901234567890_%','[^[:digit:]_,%]|.{21,}')

with
  t as (
    select 0 n, '_170_' s from dual union all
    select 0 n, '_%%' s from dual union all
    select 1, '%170_' from dual union all
    select 0, '%171' from dual union all
    select 0, '172_' from dual union all
    select 0, '173' from dual union all
    select 1, '123456789012345678_%' from dual union all
    select 0, '12345678901234567890_%' from dual
  )
select n, s, length(s) l
from t
where not regexp_like(
            s,
            q'~[^[:digit:]_%]|
               .{21,}|
               ^(\d[[:digit:]%]*|
                 \d[[:digit:]_]*|
                 _[_[:digit:]]*|
                 _[_%]*|
                 %[%[:digit:]]*|
                 %[%_]*)$~',
            'x'
          )

with
  t as (
    select 0 n, '170' s from dual union all
    select 0, '%%%' s from dual union all
    select 0, '10e6' from dual union all
    select 1, '%170%%%%%%' from dual union all
    select 1, '%%%%%%170%' from dual union all
    select 1, '%234567890123456789%' from dual union all
    select 0, '%2345678901234567890%' from dual
  )
select
  case
    when not regexp_like(s, '[^[:digit:]%]|.{21,}|^(\d\d*|%%*)$') then 1
    else 0
  end n,
  s,
  length(s) l
from t