add description and examples for pattern keyword (#108)

This PR addresses #88

content/2020-12/validation/pattern.markdown

@@ -14,3 +14,102 @@ related:
   - vocabulary: applicator
     keyword: patternProperties
 ---
+
+The `pattern` keyword in JSON Schema is designed to define a regular expression pattern that a string value within an instance must adhere to. This regular expression is specified as a string for the `pattern` keyword. It functions as follows:
+
+* Assigns a regular expression (following [ECMA-262](https://www.ecma-international.org/publications-and-standards/standards/ecma-262/) dialect) to the `pattern` keyword to define the required format.
+*  A string value is considered valid only if it successfully matches the specified pattern.
+* The regular expressions used with `pattern` are not implicitly anchored, requiring a complete match for validation. Partial matches are not accepted.
+
+## Examples
+
+{{<schema `Schema with regular expression for email validation`>}}
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "string",
+  "pattern": "^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$"
+}
+{{</schema>}}
+
+{{<instance-pass `An instance adhering to the regular expression is valid`>}}
+"[email protected]"
+{{</instance-pass>}}
+
+{{<instance-fail `An instance not adhering to the regular expression is invalid`>}}
+"invalid@yahoo"
+{{</instance-fail>}}
+
+{{<schema `Schema with regular expression for password rules`>}}
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "string",
+  "pattern": "^(?=.*[a-z])(?=.*[A-Z])(?=.*\\d)[a-zA-Z\\d]{8,}$"
+}
+{{</schema>}}
+
+{{<instance-pass `An instance adhering to the regular expression is valid`>}}
+"MyStrongPass89"
+{{</instance-pass>}}
+
+{{<instance-fail `An instance not adhering to the regular expression is invalid`>}}
+"password"
+{{</instance-fail>}}
+
+{{<schema `Schema with regular expression for usernames, including length restrictions`>}}
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "string",
+  "pattern": "^[a-zA-Z0-9_]+$",
+  "minLength": 5,
+  "maxLength": 15
+}
+{{</schema>}}
+
+{{<instance-pass `An instance with alphanumeric and underscore values, with a length between 5 and 15, is valid`>}}
+"foo_bar123"
+{{</instance-pass>}}
+
+{{<instance-fail `An instance with special character in invalid`>}}
+"invalid#username"
+{{</instance-fail>}}
+
+{{<instance-fail `An instance that matches the regex but goes out of bounds is invalid`>}}
+"username_toolong123"
+{{</instance-fail>}}
+- _This keyword can be combined with other string-related keywords, such as `maxLength` and `minLength`, for comprehensive validation._
+
+{{<schema `Schema with regular expression for some specific pattern`>}}
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "string",
+  "pattern": "apple"
+}
+{{</schema>}}
+
+{{<instance-pass `An instance matching the pattern is valid`>}}
+"apple"
+{{</instance-pass>}}
+
+{{<instance-pass `An instance is also valid if the pattern matches anywhere within the string`>}}
+"I love apples!"
+{{</instance-pass>}}
+
+- _When defining regular expressions, it's crucial to note that a string is considered valid if the expression matches anywhere within it, as demonstrated in the above example._
+
+- _To avoid this and ensure that the entire string exactly matches the `pattern`, you would surround the regular expression with `^` and `$`. See the example below._
+
+{{<schema `Schema with a regular expression enforcing an exact pattern match`>}}
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "string",
+  "pattern": "^apple$"
+}
+{{</schema>}}
+
+{{<instance-pass `An instance matching the pattern is valid`>}}
+"apple"
+{{</instance-pass>}}
+
+{{<instance-fail `An instance containing characters other than "apple" is invalid`>}}
+"I love apples!"
+{{</instance-fail>}}