substring extraction behaviour

Author: Harikrishnan-M-B

articles/frontdoor/rule-set-server-variables.md

@@ -53,6 +53,19 @@ When you work with Rule Set actions, specify server variables by using the follo
     * Offsets and lengths within range: `{var:0:5}` = `AppId`, `{var:7:7}` = `1f59297`, `{var:7:-7}` = `1f592979c584d0f9d679db3e`
     * Zero lengths: `{var:0:0}` = null,  `{var:4:0}` = null 
     * Offsets within range and lengths out of range: `{var:0:100}` = `AppId=01f592979c584d0f9d679db3e66a3e5e`, `{var:5:100}` = `=01f592979c584d0f9d679db3e66a3e5e`,  `{var:0:-48}` = null,  `{var:4:-48}` = null
+    * To experiment with how {variable:offset:length} works, open a Linux bash terminal or use bash terminal in [Azure Cloud Shell](https://shell.azure.com/). Enter the following example into the terminal and examine the output to understand how the substring extraction behaves.
+```azurecli-interactive
+>>variable=helloworld123
+>>echo ${variable:5}
+world123
+>>echo ${variable:0:4}
+hell
+>>echo ${variable: -3:3} # In bash, space comes before negative numbers.
+123
+>>echo ${variable:5: -3}
+world
+```
+ 
 * `{url_path:seg#}`: Allow users to capture and use the desired URL path segment in URL Redirect, URL Rewrite, or any meaningful action. User can also capture multiple segments by using the same style as substring capture `{url_path:seg1:3}`. For example, for a source pattern `/id/12345/default` and a URL rewrite Destination `/{url_path:seg1}/home`, the expected URL path after rewrite is `/12345/home`. For a multiple-segment capture, when the source pattern is `/id/12345/default/location/test`, a URL rewrite destination `/{url_path:seg1:3}/home` results in `/12345/default/location/home`. Segment capture includes the location path, so if route is `/match/*`, segment 0 will be match.
 
     Offset corresponds to the index of the start segment, and length refers to how many segments to capture, including the one at index = offset.