# 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) }); |
  |                |                                                              |                                                              |