README.md

@@ -123,6 +123,29 @@ from upstream. Removals must go through the whitelist.
 See [Lists](https://git.hisp.no/arr/blocklists/wiki/Lists) for the full
 rules and worked examples.
 
+## Pattern syntax
+
+Each line in `blacklist` and `whitelist` is a pattern in one of the
+following forms (the same forms that qBittorrent's excluded file names
+accepts, since that is where blacklist patterns end up via Cleanuparr's
+Blocklist Sync):
+
+| Form | Example | Matches |
+|---|---|---|
+| `*example` | `*.srt` | File name ends with `example` |
+| `example*` | `sample*` | File name starts with `example` |
+| `*example*` | `*sample*` | File name contains `example` |
+| `example` | `RARBG.txt` | File name is exactly `example` |
+| `regex:<regex>` | `regex:.*\.srt$` | File name matches the regex |
+
+The merge script subtracts the whitelist from the blacklist as **exact
+strings**, not by pattern semantics. To remove a pattern from the
+blacklist, the whitelist line must be byte-for-byte identical to the
+blacklist line. `*.srt` in the whitelist strips `*.srt` from the
+blacklist; it does **not** strip `regex:.*\.srt$` even though both
+patterns describe the same files. See
+[Lists](https://git.hisp.no/arr/blocklists/wiki/Lists) for the full rules.
+
 ## Sync schedule
 
 The Gitea Actions workflow runs every 7 days at 04:00 UTC and on manual