A regular expression, regex or regexp(sometimes called a rational expression) is a sequence of characters that define a search pattern. Usually this pattern is used by string searching algorithms for “find” or “find and replace” operations on strings, or for input validation

Regular expression - Wikipedia

Cheat sheet

Description  
abc… Letters
123… Digits
\d Any Digit
\D Any Non-digit character
. Any Character
\. Period
[abc] Only a, b, or c
[^abc] Not a, b, nor c
[a-z] Characters a to z
[0-9] Numbers 0 to 9
\w Any Alphanumeric character
\W Any Non-alphanumeric character
{n} The preceding item is matched exactly n times
{min, } The preceding item is matched min or more times
{min,max} The preceding item is matched at least min times, but not more than max times
* Zero or more repetitions
+ One or more repetitions
? Optional character
\s Any Whitespace
\S Any Non-whitespace character
^…$ Starts and ends
(…) Capture Group
(a(bc)) Capture Sub-group
(.*) Capture all
(abc|def) Matches abc or def

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