What Is a Regular Expression, Regexp, or Regex?

A regular expression is a special text string for describing a search pattern. You can think of regular expressions as wildcards on steroids. You are probably familiar with wildcard notations such as *.txt to find all text files in a file manager. The regex equivalent is .*\.txt.

But you can do much more with regular expressions. In a text editor like EditPad Pro or a specialized text processing tool like PowerGREP, you could use the regular expression \b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,6}\b to search for an email address. Anyemail address, to be exact. A very similar regular expression can be used by a programmer to check if the user entered a properly formatted email address. In just one line of code, whether that code is written in Perl, PHP, Java, a .NET language or a multitude of other languages.

Since "regular expressions" is a mouthful, you will usually find the term abbreviated as "regex" or "regexp". We prefer "regex", since it can be easily pluralized as "regexes".

What is a regular expression in programming?

Short for regular expression, a regex is a string of text that allows you to create patterns that help match, locate, and manage text. Perl is a great example of a programming language that utilizes regular expressions.

Save Time and Effort with Regex's Basically, a regular expression is a pattern describing a certain amount of text. That makes them ideally suited for searching, text processing and data validation. Searching with regular expressions enables you to get results with just one search instead of many searches. Searching for the word "separate" and all of its common misspellings is easy with the regex s[ae]p[ae]r[ae]te. If you forgot the phone number of your friend who moved to Anguilla, search your past correspondence for \b1?[-( ]*264[-) ]*\d{3}[- ]*\d{4}\b and get a handy list of all phone numbers in area code 264, regardless of the notation style used. Update large amounts of data by searching and replacing with regular expressions. The benefit of using regular expressions to update data is that with a single regex pattern, you can dynamically update a variety of data. E.g. search for (copyright +(©|\(c\)|©) +\d{4})( *[-,] *\d{4})* and replace with \1-2014 to update all copyright statements to 2014, regardless of the style of copyright symbol and the current copyright year. "copyright (c) 1996-2002" is replaced with "copyright (c) 1996-2014", while "Copyright © 2009" is replaced with "Copyright © 2009-2014", etc. This takes only a few minutes to set up and a dozen seconds to run with RegexBuddy's built-in GREP tool. Imagine making those changes by hand.

The Uses of Regex (Regular expressions) 

Regex is the gift that keeps giving. Once you learn it, you discover it comes in handy in many places where you hadn't planned to use it. On this page, we'll first look at a number of contexts and programs where you may find regex. Then we'll have a quick look at some regex flavors you may run into. Finally, we'll study some examples of regex patterns in contexts such as:

✽ File Renaming
✽ Text Search
✽ Web directives (Apache)
✽ Database queries (MySQL) 

Learn thousands of uses of regex from here Uses of The Regular Expression (Download PDF).

Quick-Start: Regex Cheat Sheet

The tables below are a reference to basic regex. While reading the rest of the site, when in doubt, you can always come back and look here. (It you want a bookmark, here's a direct link to the regex reference tables). I encourage you to print the tables so you have a cheat sheet on your desk for quick reference.

The tables are not exhaustive, for two reasons. First, every regex flavor is different, and I didn't want to crowd the page with overly exotic syntax. For a full reference to the particular regex flavors you'll be using, it's always best to go straight to the source. In fact, for some regex engines (such as Perl, PCRE, Java and .NET) you may want to check once a year, as their creators often introduces new features. 

Download and use regex cheat sheet and start learning regular expression  Regular Expression Cheat Sheet (Download PDF).