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+# REGEX Quick Guide
+
+* [Regex 101](https://regex101.com/) : Have sample code generator
+
+* `Atom` and `Notepad++` support Regex `Find` / `Replace`. Can use to test regex expression
+
+* Javascript : Enclosed within `/.../`
+
+* Control characters
+
+  * `.` : Any Character Except New Line
+  * `\d` : Digit (0-9)
+  * `\D` : Not a Digit
+  * `\w` : Word Character (a-z, A-Z, 0-9, _)
+  * `\W` : Not Word Character
+  * `\s` : Whitespace (space, tab, newline)
+  * `\S` : Not Whitespace
+  * `\b` : Word Boundary
+  * `\B` : Not Word Boundary
+  * `^` : Beginning of a String
+  * `$` : End of a String
+
+* Other
+
+  * `[]` : Matches Characters in brackets
+  * `[^ ]` : Matches Characters NOT in brackets
+  * `|` : Either Or
+  * `( )` : Group
+
+* Quantifiers
+
+  * `*` : 0 or more
+  * `+` : 1 or more
+  * `?` : 0 or One
+  * `{3}` : Exact Number
+  * `{3,4}` : Range of Numbers (Minimum, Maximun)
+
+* MetaCharacters (Need to be escaped)
+
+  * `.[{()\^$|?*+` : Need to be escaped with `\`. 
+  * Eg  String = `\*`  
+    * To form the character list, escape it as shown `\\\*`
+
+* Useful regex
+
+  | regex          | Description                                                  | Sample javascript                                            |
+  | -------------- | ------------------------------------------------------------ | ------------------------------------------------------------ |
+  | /\d(?=\d{4})/g | mask digits and keep remaining 4 digits                      | '12345678'.replace(/\d(?=\d{4})/g, "*")                      |
+  | /(\d+)(\d{4})/ | mask digit. Group into group1 (length - 4 digits) and group2 (4-digit) | '12345678'.replace(/(\d+)(\d{4})/, (match, g1, g2) => { return g1.replace(/\d+/, '*'.repeat(g1.length) + g2) }); |
+  |                |                                                              |                                                              |
+
+  
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