A regular expression (also called regex) is a way to work with strings, in a very performant way.

By formulating a regular expression with a special syntax, you can

• search text in a string
• replace substrings in a string
• extract information from a string

Almost every programming language implements regular expressions. There are small differences between each implementation, but the general concepts apply almost everywhere.

Regular Expressions date back to the 1950s, when it was formalized as a conceptual search pattern for string processing algorithms.

Implemented in UNIX tools like grep, sed, and in popular text editors, regexes grew in popularity and were introduced in the Perl programming language, and later in many others.

JavaScript, among with Perl, is one of the programming languages that have regular expressions support directly built in the language.

Regular expressions can appear like absolute nonsense to the beginner, and many times also to the professional developer, if one does not invest the time necessary to understand them.

Cryptic regular expressions are hard to write, hard to read, and hard to maintain/modify.

But sometimes a regular expression is the only sane way to perform some string manipulation, so it’s a very valuable tool in your pocket.

This tutorial aims to introduce you to JavaScript Regular Expressions in a simple way, and give you all the information to read and create regular expressions.

The rule of thumb is that simple regular expressions are simple to read and write, while complex regular expressions can quickly turn into a mess if you don’t deeply grasp the basics.

In JavaScript, a regular expression is an object, which can be defined in two ways.

The first is by instantiating a new RegExp object using the constructor:

const re1 = new RegExp('hey')

The second is using the regular expression literal form:

const re1 = /hey/

You know that JavaScript has object literals and array literals? It also has regex literals.

In the example above, hey is called the pattern. In the literal form it’s delimited by forward slashes, while with the object constructor, it’s not.

This is the first important difference between the two forms, but we’ll see others later.

The regular expression we defined as re1 searches the string hey, without any limitation: the string can contain lots of text, and hey in the middle, and the regex is satisfied. It could also contain just hey, and it will be satisfied as well.

You can test the regex using RegExp.test(String), which returns a boolean:

re1.test('hey')                     //✅
re1.test('blablabla hey blablabla') //✅


re1.test('he')        //❌
re1.test('blablabla') //❌

In the above example we just checked if "hey" satisfies the regular expression pattern stored in re1.

This is the simplest it can be, but you already know lots of concepts about regexes.

Lessons in this unit:

0:	Introduction
1:	▶︎ Introduction
2:	Anchoring
3:	Match Items in Ranges
4:	Matching a Range Item Multiple Times
5:	Negating a Pattern
6:	Meta Characters
7:	Regular Expressions Choices
8:	Quantifiers
9:	Optional Items
10:	Groups
11:	Capturing Groups
12:	Using match and exec Without Groups
13:	Noncapturing Groups
14:	Flags
15:	Inspecting a Regex
16:	Escaping
17:	String Boundaries
18:	Replacing
19:	Greediness
20:	Lookaheads
21:	Lookbehinds
22:	Unicode
23:	Unicode Property Escapes
24:	Examples