psql regular expression matching POSIX

Could someone please provide me with the syntax for a REGEXP that will
match a column in a table?

ie, this works using "similar to":
   select prod_id, prod_desc, vend_id from products where\
       vend_id similar to '\[D|F]%'


In the docs:
   pgsql/doc/html/functions-matching.html  para 9.7.3

I'm suppose to be able to use POSIX regular expressions.

Say I want to match all the vendor IDs with "LL" located anywere
inside the vend_id.

    select vend_id from products where <Now What???>

All of the examples in this section discuss "select substring"

All the variations I've tried of the following either are
syntax errors or 'type "vend_id" does not exist'.
   SELECT vend_id from products  where  substring (vend_id '.*LL.*');

Thanks