Well, I ended up aborting the Rails process and went back to the pg console. I also received a suggestion for a much more simple (and hopefully more easy to debug) regexp that also works very well outside of the pg console:
"(.*?)":(\S*)\b
But there’s definitely something non-standard about the SQL query, this did nothing at all to the links:
update posts set raw = regexp_replace(raw, '"(.*?)":(\S*)\b', E'[\\1](\\2)', 'g');
Nor did this:
update posts set raw = regexp_replace(raw, '"(.*?)"\:(\S*)\b', E'[\\1](\\2)', 'g');
So what gives?