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====== RegEx ======

**Regular Expressions**

[[https://cheatography.com/davechild/cheat-sheets/regular-expressions/|Good Cheat Sheet]] | [[https://medium.com/factory-mind/regex-tutorial-a-simple-cheatsheet-by-examples-649dc1c3f285|Basic Explanation]]


===== Anchors =====

^  Syntax  ^ Description                                             ^
|  %%^%%   | Start of string, or start of line in multi-line pattern |
|  $       | End of string, or end of line in multi-line pattern     |

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===== Quantifiers =====

^  Syntax    ^ Description                                 ^
|  *         | 0 or more                                   |
|  ?         | 0 or 1                                      |
|  +         | 1 or more                                   |
|  {3}       | Exactly 3                                   |
|  {3,}      | 3 or more                                   |
|  {3,5}     | 3, 4 or 5                                   | 
|  //note//  | Add a ? to a quantifier to make it ungreedy |

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===== Escape =====

^  Syntax  ^ Description                ^
|  \       | Escape following character |

**//Escaping//** is a way of treating characters which have a special meaning in regular expressions literally, rather than as special characters.

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===== Groups and Ranges =====

^  Syntax      ^ Description                        ^
|  .           | Any character except new line (\n) |
|  (a%%|%%b)   | a or b                             |
|  (...)       | Group, capturing                   |
|  (?:...)     | Passive (non-capturing) group      | 
|  [abc]       | Range (a or b or c)                |
|  [%%^%%abc]  | Not (a or b or c)                  | 
|  [a-q]       | Lower case letter from a to q      |
|  [A-Q]       | Upper case letter from A to Q      | 
|  [0-7]       | Digit from 0 to 7                  |

Use a group to "capture and replace".
  * ''a(bc)'' parentheses create a capturing group with value ''bc''
  * The parentheses are used to create "groups," which then get assigned a base-1 index, accessible in a //replace// with a ''$'' (i.e., the first parentheses-bounded (characters) become a group that can be referenced by ''$1'', the second group would be ''$2'', the third ''$3'', etc.).
  * NOTE: in **gedit** you reference with a backslash: ''\'' not a ''$'' (i.e., ''\1\2\3'', etc.).
  * See the follow table regarding **string replacements**.

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===== String Replacement =====

^  Syntax      ^ Description           ^
|  $n  | nth non-passive group         |
|  $2  | %%"xyz" in /^(abc(xyz))$/%%   |
|  $1  | %%"xyz" in /^(?:abc)(xyz)$/%% |
|  $`  | Before matched string         |
|  $'  | After matched string          |
|  $+  | Last matched string           |
|  $&  | Entire matched string         |

**Note:** Some regex implementations use \ instead of $.

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===== Character Classes =====

^  Syntax  ^ Description   ^
|  \c  | Control character |
|  \s  | White space       |
|  \S  | Not white space   |
|  \d  | Digit             |
|  \D  | Not digit         |
|  \w  | Word              |
|  \W  | Not word          |
|  \x  | Hexadecimal digit |
|  \O  | Octal digit       |

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===== POSIX =====

^  Syntax      ^ Description                    ^
|  [:upper:]   | Upper case letters             |
|  [:lower:]   | Lower case letters             |
|  [:alpha:]   | All letters                    |
|  [:alnum:]   | Digits and letters             |
|  [:digit:]   | Digits                         |
|  [:xdigit:]  | Hexadecimal digits             |
|  [:punct:]   | Punctuation                    |
|  [:blank:]   | Space and tab                  |
|  [:space:]   | Blank characters               |
|  [:cntrl:]   | Control characters             |
|  [:graph:]   | Printed characters             |
|  [:print:]   | Printed characters and spaces  |
|  [:word:]    | Digits, letters and underscore |

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