====== Regex ======

----

=== Characters ===
|\d |one digit from 0 to 9|
|\w |word character: ASCII letter, digit or underscore|
|\s |whitespace character: space, tab, newline, carriage return, vertical tab|
|\D |One character that is not a digit|
|\W |One character that is not a word character )|
|\S |One character that is not a whitespace character |
|\metachar |Escape a metacharacter "%%^{+<[*)>.(|$\?|%%" |
|\t |Tab|
|\r |Carriage return character|
|\n |Line feed character|
|\r\n |Line separator|

=== Quantifiers ===
|a? |Zero or one of a|
|a* |Zero or more of a|
|a+ |One or more of a|
|[0-9]+ |One or more of 0-9|
|a{3} |Exactly 3 of a|
|a{3,} |3 or more of a|
|a{3,6} |Between 3 and 6 of a|
|a* |Greedy quantifier|
|a*? |Lazy quantifier|
|a*+ |Possessive quantifier|

=== Character Classes ===
|[abc] |A single character of: a, b or c|
|[%%^%%abc] |A character except: a, b or c|
|[a-z] |A character in the range: a-z|
|[%%^%%a-z] |A character not in the range: a-z|
|[0-9] |A digit in the range: 0-9|
|[a-zA-Z] |A character in the range: a-z or A-Z|
|[a-zA-Z0-9] |A character in the range: a-z, A-Z or 0-9|
|[\x41] |A character at hex position 41 in the ASCII table, i.e. A|

=== Anchors ===
|%%^%% |Start of string|
|$ |End of string|
|\A |Start of string|
|\Z |End of string|
|\z |Absolute end of string|
|\b |A word boundary|
|\B |Non-word boundary|
|\G |Start of match|

=== Group Constructs ===
|(...) |Capturing group: Match everything enclosed|
|(a%%|%%b) |Capturing group: Match either a or b|
|(?P<Y>...)|Capturing group named Y|
|(?:...) |Non-capturing group: Matches and does not remember the match|
|(?>...) |Atomic group (non-capturing)|
|(?%%|%%...) |Duplicate subpattern group number|

=== Lookarounds ===
|(?=...) |Positive Lookahead. x(?=y)Matches "x" only if "x" is followed by "y"|
|(?!...) |Negative Lookahead. x(?!y) Matches "x" only if "x" is followed not by "y"|
|(?%%<%%=...) |Positive Lookbehind (?<=y)x Matches "x" only if "x" is preceded by "y"|
|(?%%<%%!...) |Negative Lookbehind (?<!y)x Matches "x" only if "x" is not preceded by "y"|

==== Cheat sheets ====
  * [[https://www.rexegg.com/regex-quickstart.html#classes]]
  * [[https://quickref.me/regex.html]]
  * [[https://www.datacamp.com/cheat-sheet/regular-expresso]]

==== Online Testers ====
  * [[https://www.regextester.com/|Regextester]]
  * [[https://regexr.com/|Regexr]]

==== Links ====
  * [[http://www.fail2ban.org/wiki/index.php/MANUAL_0_8#Filters|Fail2ban Regex]]