Splitting long lines in a nginx configuration file
I don't think you can do this.
nginx treats all whitespace equally, so even if you tried to split your string like that, and nginx could parse it the way you intended, you would wind up with a regex with a bunch of whitespace in it, which I'm sure isn't what you want. If nginx couldn't parse it, which is more likely, you'd just get a syntax error.
You're just going to have to live with a few long lines, or make less complex regular expressions.