Regular Expressions are fancy wildcards. Typically abbreviated "regex", they allow you to find / match as well as replace inexact patterns and even special characters (like tabs and line breaks) in text. This is useful in many programming languages, but also for finding-and-replacing in documents. Most programming languages and many text editors, including Microsoft Word (2010+) allow the use of standard regex symbols and syntax. There are some differences in how regular expressions work in different software, but much of the basics are identical. The term "grep" which is used by programmers synonymous with search, stands for "global regular expression print".
Regular Expressions and Wildcards can be used for finding and replacing or removing text in a file, database, or filename. It is very powerful and can even assist in cleaning and reformatting data.
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The vast majority of RegEx symbols are exactly the same across software, so these resources are broadly useful.
|Stata||What are regular expressions and how can I use them in Stata? (STATA)
More examples: How can I extract a portion of a string variable using regular expressions? (UCLA)
|R||Finding: Regular Expression in R (Gloria Li and Jenny Bryan)
Extracting Extract information from texts with regular expressions in R (Kun Ren)
|Python||Regular Expression HOW TO and Operations (Python Documentation)
Text Processing in Python by David Mertz -- related: Matching Patterns in Text: The Basics
|Notepad++||How to use regular expressions in Notepad++ (NP++ Wiki)
A guide to using regular expressions and extended search mode (Mark Antoniou)
|Atom||Find and Replace (Atom Manual)
Regular expressions in Atom for code consistency (Modelit)
|Office||Microsoft: Finding and replacing characters using wildcards (Graham Mayor, MVP)
OpenOffice: Regular Expressions in Writer (OpenOffice Documentation)
|Perl||Perl Regular Expression (Perl Documentation)
Teach Yourself Perl in 21 Days by Laura Lemay - excerpt: Pattern Matching
|SQL||SQL Wildcards (W3 Schools)|