Add docs on pattern matching syntax in variables expression

doc/ci/variables/README.md

@@ -530,6 +530,13 @@ Below you can find supported syntax reference:
     `$STAGING` value needs to a string, with length higher than zero.
     Variable that contains only whitespace characters is not an empty variable.
+1. Pattern matching  _(added in 11.0)_
+
+    > Example: `$VARIABLE =~ /^content.*/`
+
+    It is possible perform pattern matching against a variable and regular
+    expression. Expression like this evaluates to truth if matches are found.
+
 ### Unsupported predefined variables
 Because GitLab evaluates variables before creating jobs, we do not support a

doc/ci/yaml/README.md

@@ -344,10 +344,11 @@ job:
     kubernetes: active
 ```
-Example of using variables expressions:
+Examples of using variables expressions:
 ```yaml
 deploy:
+  script: cap staging deploy
   only:
     refs:
       - branches
@@ -356,6 +357,16 @@ deploy:
       - $STAGING
 ```
+Another use case is exluding jobs depending on a commit message _(added in 11.0)_:
+
+```yaml
+end-to-end:
+  script: rake test:end-to-end
+  except:
+    variables:
+      - $CI_COMMIT_MESSAGE =~ /skip-end-to-end-tests/
+```
+
 Learn more about variables expressions on [a separate page][variables-expressions].
 ## `tags`