Compress 2001:0db8:0000:0000:0000:ff00:0042:8329 as RFC 5952 asks.
Answer
2001:db8::ff00:42:8329
Both conventions apply: leading zeros within each hextet are dropped, and the longest run of all-zero hextets collapses to ::. Lowercase hex is part of the recommendation, which rules out the uppercase form. The first option drops leading zeros but leaves the zero run written out, and the third does the reverse — compressing the run while keeping 0db8 and 0042 padded. All four denote the same address; the point of the rule is that one address gets written the same way in a log, a config file, and a firewall rule, so a search for it finds it.