--- a/toolkit/components/telemetry/docs/collection/events.rst
+++ b/toolkit/components/telemetry/docs/collection/events.rst
@@ -48,28 +48,37 @@ Where the individual fields are:
 - ``value``: ``String``, optional, may be ``null``. This is a user defined value, providing context for the event.
 - ``extra``: ``Object``, optional, may be ``null``. This is an object of the form ``{"key": "value", ...}``, both keys and values need to be strings, keys are identifiers. This is used for events where additional richer context is needed.
 
 .. _eventlimits:
 
 Limits
 ------
 
-Each ``String`` marked as an identifier is restricted to the following regex pattern: ``^[:alpha:][:alnum:_.]*[:alnum:]$``.
+Each ``String`` marked as an identifier (the event ``name``, ``category``, ``method``,
+``object``, and the keys of ``extra``) is restricted to be composed of alphanumeric ASCII
+characters ([a-zA-Z0-9]) plus infix underscores ('_' characters that aren't the first or last).
+``category`` is also permitted infix periods ('.' characters, so long as they aren't the
+first or last character).
 
-For the Firefox Telemetry implementation, several fields are subject to limits:
+For the Firefox Telemetry implementation, several fields are subject to length limits:
 
 - ``category``: Max. byte length is ``30``.
 - ``method``: Max. byte length is ``20``.
 - ``object``: Max. byte length is ``20``.
 - ``value``: Max. byte length is ``80``.
 - ``extra``: Max. number of keys is ``10``.
+
   - Each extra key name: Max. string length is ``15``.
   - Each extra value: Max. byte length is ``80``.
 
+Only ``value`` and the values of ``extra`` will be truncated if over the specified length.
+Any other ``String`` going over its limit will be reported as an error and the operation
+aborted.
+
 The YAML definition file
 ========================
 
 Any event recorded into Firefox Telemetry must be registered before it can be recorded.
 For any code that ships as part of Firefox that happens in `Events.yaml <https://dxr.mozilla.org/mozilla-central/source/toolkit/components/telemetry/Events.yaml>`_.
 
 The probes in the definition file are represented in a fixed-depth, three-level structure. The first level contains *category* names (grouping multiple events together), the second level contains *event* names, under which the events properties are listed. E.g.:
 
@@ -90,17 +99,17 @@ The probes in the definition file are re
     completion:
       # ...
       description: Recorded when a search completion suggestion was clicked.
       extra_keys:
         distance: The edit distance to the current search query input.
         loadtime: How long it took to load this completion entry.
     # ...
 
-Category and probe names are subject to the limits and regex patterns :ref:`specified above <eventlimits>`.
+Category and event names are subject to the limits :ref:`specified above <eventlimits>`.
 
 The following event properties are valid:
 
 - ``methods`` *(optional, list of strings)*: The valid event methods. If not set this defaults to ``[eventName]``.
 - ``objects`` *(required, list of strings)*: The valid event objects.
 - ``description`` *(required, string)*: Description of the event and its semantics.
 - ``release_channel_collection`` *(optional, string)*: This can be set to ``opt-in`` (default) or ``opt-out``.
 - ``record_in_processes`` *(required, list of strings)*: A list of processes the event can be recorded in. Currently supported values are: