Attempt to clarify how event_match works (#3690)

Fixes #3082, #2637, #3075.

changelogs/client_server/3690.clarification

@@ -0,0 +1 @@
+Clarify the behaviour of `event_match` in push rule conditions.

content/client-server-api/modules/push.md

@@ -211,25 +211,114 @@ other keys as their parameters, e.g.
 
 `override` and `underride` rules MAY have a list of 'conditions'. All
 conditions must hold true for an event in order for the rule to match. A
-rule with no conditions always matches. The following conditions are
-defined:
+rule with no conditions always matches.
+
+Unrecognised conditions MUST NOT match any events, effectively making
+the push rule disabled.
+
+`room`, `sender` and `content` rules do not have conditions in the same
+way, but instead have predefined conditions. In the cases of `room` and
+`sender` rules, the `rule_id` of the rule determines its behaviour.
+
+The following conditions are defined:
+
+**`event_match`**
 
-`event_match`
 This is a glob pattern match on a field of the event. Parameters:
 
--   `key`: The dot-separated field of the event to match, e.g.
-    `content.body`
--   `pattern`: The glob-style pattern to match against. Patterns with no
-    special glob characters should be treated as having asterisks
-    prepended and appended when testing the condition.
+-   `key`: The dot-separated path of the property of the event to match, e.g.
+    `content.body`.
+
+-   `pattern`: The glob-style pattern to match against.
+
+The match is performed case-insensitively, and must match the entire value of
+the event field given by `key` (though see below regarding `content.body`). The
+exact meaning of "case insensitive" is defined by the implementation of the
+homeserver.
+
+Within `pattern`:
+
+  * The character `*` matches zero or more characters.
+  * `?` matches exactly one character.
+
+If the property specified by `key` is completely absent from the event, or does
+not have a string value, then the condition will not match, even if `pattern`
+is `*`.
+
+{{% boxes/note %}}
+For example, if `key` is `content.topic`, and `pattern` is `lunc?*`, then
+the following event will match:
+
+```json
+{
+  "content": {
+    "topic": "Lunch plans",
+  },
+  "event_id": "$143273582443PhrSn:example.org",
+  "room_id": "!636q39766251:example.com",
+  "sender": "@example:example.org",
+  "state_key": "",
+  "type": "m.room.topic"
+}
+```
+
+Other `topic` values which will match are:
+
+ * `"LUNCH"` (case-insensitive; `*` may match zero characters)
+
+The following `membership` values will NOT match:
+ * `" lunch"` (note leading space)
+ * `"lunc"` (`?` must match a character)
+ * `null` (not a string)
+{{% /boxes/note %}}
+
+As a special case, if `key` is `content.body`, then `pattern` must instead
+match any substring of the value of the property which starts and ends at a
+word boundary. A word boundary is defined as the start or end of the value, or
+any character not in the sets `[A-Z]`, `[a-z]`, `[0-9]` or `_`.
+
+{{% boxes/note %}}
+For example, if `key` is `content.body` and `pattern` is `ex*ple`, the
+following event will match:
+
+```json
+{
+  "content": {
+    "body": "An example event.",
+  },
+  "event_id": "$143273976499sgjks:example.org",
+  "room_id": "!636q39766251:example.com",
+  "sender": "@example:example.org",
+  "type": "m.room.message"
+}
+```
+
+Other `body` values which will match are:
+
+ * `"exple"` (the pattern can match at the start and end of the body.)
+ * `"An exciting triple-whammy"` (the pattern can span multiple words, and `-`
+   acts as a word separator.)
+{{% /boxes/note %}}
+
+
+{{% boxes/warning %}}
+Note that there is no implicit condition for `state_key`. In other words, push
+rules which should match only state events must include an explicit condition
+for `state_key`.
+
+For an example of this, see the default rule
+[`.m.rule.tombstone`](#mruletombstone) below.
+{{% /boxes/warning %}}
+
+**`contains_display_name`**
 
-`contains_display_name`
 This matches unencrypted messages where `content.body` contains the
 owner's display name in that room. This is a separate rule because
 display names may change and as such it would be hard to maintain a rule
 that matched the user's display name. This condition has no parameters.
 
-`room_member_count`
+**`room_member_count`**
+
 This matches the current number of members in the room. Parameters:
 
 -   `is`: A decimal integer optionally prefixed by one of, `==`, `<`,
@@ -237,7 +326,8 @@ This matches the current number of members in the room. Parameters:
     count is strictly less than the given number and so forth. If no
     prefix is present, this parameter defaults to `==`.
 
-`sender_notification_permission`
+**`sender_notification_permission`**
+
 This takes into account the current power levels in the room, ensuring
 the sender of the event has high enough power to trigger the
 notification.
@@ -251,13 +341,6 @@ Parameters:
     to look up the power level required to send a notification type from
     the `notifications` object in the power level event content.
 
-Unrecognised conditions MUST NOT match any events, effectively making
-the push rule disabled.
-
-`room`, `sender` and `content` rules do not have conditions in the same
-way, but instead have predefined conditions. In the cases of `room` and
-`sender` rules, the `rule_id` of the rule determines its behaviour.
-
 ##### Predefined Rules
 
 Homeservers can specify "server-default rules" which operate at a lower
@@ -403,7 +486,7 @@ Definition:
 }
 ```
 
-**`.m.rule.tombstone`**
+**<a name="mruletombstone"></a>`.m.rule.tombstone`**
 
 Matches any state event whose type is `m.room.tombstone`. This is
 intended to notify users of a room when it is upgraded, similar to what

data/api/client-server/definitions/push_condition.yaml

@@ -35,9 +35,7 @@ properties:
     type: string
     description: |-
       Required for `event_match` conditions. The glob-style pattern to
-      match against. Patterns with no special glob characters should be
-      treated as having asterisks prepended and appended when testing the
-      condition.
+      match against.
   is:
     type: string
     description: |-