Added an example for using regex (#12822)

* Added an example for using regex

This is a working example from my config. Hopefully can save someone some time with the general syntax.

* Update entity-filter.markdown

* Minor changes

Co-authored-by: Fabian Affolter <mail@fabian-affolter.ch>

source/_lovelace/entity-filter.markdown

@@ -94,7 +94,8 @@ attribute:
 
 ### Examples
 
-Show only active switches or lights in the house
+Show only active switches or lights in the house.
+
 ```yaml
 type: entity-filter
 entities:
@@ -120,12 +121,13 @@ card:
   type: glance
   title: People at home
 ```
+
 <p class='img'>
   <img src='/images/lovelace/lovelace_entity_filter_glance.png' alt='Entity filter combined with glance card'>
   Entity filter combined with glance card.
 </p>
 
-You can also specify multiple state_filters, in which case matching any condition will display the entity. This example will display everyone who isn't at home or at work.
+You can also specify multiple `state_filter`s, in which case matching any condition will display the entity. This example will display everyone who isn't at home or at work.
 
 ```yaml
 type: entity-filter
@@ -143,7 +145,7 @@ card:
   title: Who's Running Errands
 ```
 
-Specify filter for a single entity
+Specify filter for a single entity.
 
 ```yaml
 type: entity-filter
@@ -160,3 +162,23 @@ entities:
         value: 50
         attribute: humidity
 ```
+
+Use a regex filter against entity attributes. This regex filter below looks for expressions that are 1 digit in length and where the number is between 0-7 (so show holidays today or in the next 7 days) and displays those holidays as entities in the entity-filter card.
+
+```yaml
+  - type: entity-filter
+    card:
+      title: "Upcoming Holidays In Next 7 Days"
+      show_header_toggle: false
+    state_filter:
+      - operator: regex
+        value: "^([0-7]{1})$"
+        attribute: eta
+    entities:
+      - entity: sensor.upcoming_ical_holidays_0
+      - entity: sensor.upcoming_ical_holidays_1
+      - entity: sensor.upcoming_ical_holidays_2
+      - entity: sensor.upcoming_ical_holidays_3
+      - entity: sensor.upcoming_ical_holidays_4
+    show_empty: false
+```