Docs update

Author: vivekkpatel

docs/Regular Expressions.md

@@ -1,3 +1,40 @@
+# Regular Expression
+
+## What it is?
+- It's pattern matching language used to find and extract text
+- for example to find/match all dates in text we can use `\d{4}-\d{2}-\d{2}` format to match Data: YYYY-MM-DD pattern
+- Use Case
+  - Check validity fo user input
+    - like if user given input is valid email id, licensekey)
+    - If given data contain specific info like string contain numbers or symbols
+  - Data extraction using pattern matching
+
+## How to use it?
+- Use [Regexr](https://regexr.com/) website for testing
+- Here is sample code to find all numbers from string
+- If we only use `\d` it will print individual number separately like `4,2,5,0,0,9,9,9`
+- That extra `+` sign allows whole number till next space
+
+```csharp
+string text = "My numbers are 42, 500 and 999";
+string pattern = @"\d+";
+
+MatchCollection matches = Regex.Matches(text, pattern);
+
+foreach (Match m in matches)
+{
+    Console.WriteLine(m.Value);
+}
+```
+```
+42
+500
+999
+```
+
+
+## Pattern
+
 ```
 .       - Any Character Except New Line
 \d      - Digit (0-9)