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What is Regex?

Regular expressions are notations for describing patterns of text and, in effect, make up a special-purpose language for pattern matching. Although there are myriad variants, all share the idea that most characters in a pattern match literal occurrences of themselves, but some metacharacters have special meaning, such as * to indicate some kind of repetition or […] to mean any one character from the set within the brackets.

Beautiful Code by Andy Oram, Greg Wilson

More simply, Regex (short for regular expression), is a string of text that allows you to create patterns that help match, locate, and manage text.

Below is a quick reference Javascript regex cheat sheet. If you need a more in depth refresher or a place to get started I recommend these resources on regex:

Language	Reference Materials
Javacript	Regular expressions – JavaScript | MDN (mozilla.org) JavaScript RegExp Reference (w3schools.com)
T-SQL (MS SQL Server)	Search Text with Regular Expressions – SQL Server Management Studio (SSMS) | Microsoft Docs
Python	Regular Expression HOWTO — Python 3.9.6 documentation Python RegEx (w3schools.com)
C Languages	Regular expressions in C – GeeksforGeeks An example of using regular expressions in C (lemoda.net) Regex Class (System.Text.RegularExpressions) | Microsoft Docs (C#)
Pearl (PCRE)	perlre – Perl regular expressions – Perldoc Browser
Powershell	about Regular Expressions – PowerShell | Microsoft Docs
Java	Java Regular Expressions (w3schools.com) Lesson: Regular Expressions (The Java™ Tutorials > Essential Java Classes) (oracle.com)
Linux (Bash)	How to Use Regular Expressions (regexes) on Linux (howtogeek.com) Using Grep + Regex (Regular Expressions) to Search Text in Linux | DigitalOcean Advanced Bash regex with examples – Linux Tutorials – Learn Linux Configuration
Other Resources and Reference Material	Comparison of regular-expression engines – Wikipedia An introduction to regular expressions – O’Reilly (oreilly.com)

Quick Reference Cheat Sheet

Character classes
.	any character except newline
\w\d\s	word, digit, whitespace
\W\D\S	not word, digit, whitespace
[abc]	any of a, b, or c
[^abc]	not a, b, or c
[a-g]	character between a & g
Anchors
^abc$	start / end of the string
\b\B	word, not-word boundary
Escaped characters
\.\*\\	escaped special characters
\t\n\r	tab, linefeed, carriage return
Groups & Lookaround
(abc)	capture group
\1	backreference to group #1
(?:abc)	non-capturing group
(?=abc)	positive lookahead
(?!abc)	negative lookahead
Quantifiers & Alternation
a*a+a?	0 or more, 1 or more, 0 or 1
a{5}a{2,}	exactly five, two or more
a{1,3}	between one & three
a+?a{2,}?	match as few as possible
ab|cd	match ab or cd

Regex Tester Tool

Disclaimer: For educational and demonstrative purposes only. Always test your regular expressions before applying anything to a production system. Results from the above tool are not guaranteed.