Adds a section on input validation

pages/pipelines/input_step.md

@@ -275,3 +275,22 @@ steps:
             value: "stable"
 ```
 {: codeblock-file="pipeline.yml"}
+
+## Input validation
+
+To prevent users from entering invalid text values in input steps  (for example, to gather some deployment information), you can use input validation.
+
+If you associate a regular expression to a field, the field outline will turn red when an invalid value is entered.
+
+To do it, use the following sample syntax:
+
+```yml
+steps:
+  - input: "Click me!"
+    fields:
+      - text: Must be hexadecimal
+        key: hex
+        format: "[0-9a-f]+"
+```
+
+The `format` must be a regular expression implicitly anchored to the beginning and end of the input and is functionally equivalent to the [HTML5 pattern attribute](https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/pattern).