Validate date format oracle

I would like to validate date column of a table with format 'YYYY-MM-DD HH24:MM:SS'. The query i used so far is

select count(*) from table-name where not regexp-like(datefield,'/^(\d{4})-(\d{2})-(\d{2}) (\d{2}):(\d{2}):(\d{2})$/'

I would like to validate using a different method for the above requirement, like using sysdate format. Could someone please help me.

Thanks