select regexp_matches('Hi, I am Harry Potter', '^[a-zA-Z0-9]*\W+\w+');
select regexp_matches('Hi, I am Harry Potter', '\w+\W+\w+');
Both way returns {Hi, I}
But expect {Hi I}.
Related Question: extract first word in an String: extract the first word from a string - regex
You cannot match disjoint (non-adjoining) parts of a string into a single group.
You can use REGEXP_REPLACE to capture the first two words into separate groups and then use two backreferences to the group in the replacement pattern to get what you need:
select regexp_replace('Hi, I am Harry Potter', '^\W*(\w+)\W+(\w+).*', '\1 \2');
See the online demo. The regex means
^ - start of string\W* - zero or more non-word chars(\w+) - Group 1 (\1): one or more word chars\W+ - one or more non-word chars(\w+) - Group 2 (\2): one or more word chars.* - the rest of the string.
You can use this pattern:
select regexp_match(
'Hi, I am Harry Potter',
'^([[:alnum:]]+)[^[:alnum:]]+([[:alnum:]]+)'
);
regexp_matches
════════════════
{Hi,I}
(1 row)
The pattern matches the first sequence of alphanumerical characters, then a sequence of non-alphanumerical characters and another sequence of alphanumerical characters. The result is an array with the first and third expression, which are parenthesized in the pattern.
