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WelcomeRegular Expressions Quick StartRegular Expressions TutorialReplacement
Strings TutorialApplications and LanguagesRegular Expressions ExamplesRegular
Expressions ReferenceReplacement Strings ReferenceBook ReviewsPrintable PDFAbout
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WELCOME TO REGULAR-EXPRESSIONS.INFO
THE PREMIER WEBSITE ABOUT REGULAR EXPRESSIONS

A regular expression (regex or regexp for short) 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,}\b to search for an
email address. Any email address, to be exact. A very similar regular expression
(replace the first \b with ^ and the last one with $) can be used by a
programmer to check whether 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.

REGULAR EXPRESSIONS QUICK START

If you just want to get your feet wet with regular expressions, take a look at
the one-page regular expressions quick start. While you can’t learn to
efficiently use regular expressions from this brief overview, it’s enough to be
able to throw together a bunch of simple regular expressions. Each section in
the quick start links directly to detailed information in the tutorial.

COMPLETE REGULAR EXPRESSIONS TUTORIAL

Do not worry if the above example or the quick start make little sense to you.
Any non-trivial regex looks daunting to anybody not familiar with them. But with
just a bit of experience, you will soon be able to craft your own regular
expressions like you have never done anything else. The free
Regular-Expressions.info Tutorial explains everything bit by bit.

This tutorial is quite unique because it not only explains the regex syntax, but
also describes in detail how the regex engine actually goes about its work. You
will learn quite a lot, even if you have already been using regular expressions
for some time. This will help you to understand quickly why a particular regex
does not do what you initially expected, saving you lots of guesswork and head
scratching when writing more complex regexes.

REPLACEMENT STRINGS TUTORIAL

A replacement string, also known as the replacement text, is the text that each
regular expression match is replaced with during a search-and-replace. In most
applications, the replacement text supports special syntax that allows you to
reuse the text matched by the regular expression or parts thereof in the
replacement. This website also includes a complete replacement strings tutorial
that explains this syntax. While replacement strings are fairly simple compared
with regular expressions, there is still great variety between the syntax used
by various applications and their actual behavior.

APPLICATIONS & LANGUAGES THAT SUPPORT REGEXES

There are many software applications and programming languages that support
regular expressions. If you are a programmer, you can save yourself lots of time
and effort. You can often accomplish with a single regular expression in one or
a few lines of code what would otherwise take dozens or hundreds.

Many applications and programming languages have their own implementation of
regular expressions, often with slight and sometimes with significant
differences from other implementations. When two applications use a different
implementation of regular expressions, we say that they use different “regular
expression flavors”. Unlike most other regex tutorials, the tutorial on this
website covers all the popular regular expression flavors, and indicates the
differences that you should watch out for.

NOT ONLY FOR PROGRAMMERS

If you are not a programmer, you can use regular expressions in many situations
just as well. They make finding information a lot easier. You can use them in
powerful search and replace operations to quickly make changes across large
numbers of files. A simple example is gr[ae]y which finds both spellings of the
word gray in one operation, instead of two. There are many text editors and
search and replace tools with decent regex support.

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