substring odd behavior

Is this intentional behavior?

-- I can do this
SELECT substring('3.2.0' from '[0-9]*\.([0-9]*)\.'); 

-- But can't do this gives error syntax error at or near "from"
SELECT pg_catalog.substring('3.2.0' from '[0-9]*\.([0-9]*)\.');

-- but can do
SELECT pg_catalog.substring('3.2.0', '[0-9]*\.([0-9]*)\.');


Thanks,
Regina