What Are Regular Expressions in JavaScript?

A regular expression (regex) is a pattern that describes text. Instead of asking “does this string contain exactly cat?”, you can ask “does it contain a word that starts with a capital letter and ends with a number?”

Use cases you will meet:

• checking that an input looks like an email, a phone number or a postcode,
• finding or replacing patterns in text,
• pulling parts out of a string, such as the digits in "Order #4521".

Creating a regex

Two ways. The literal form uses slashes:

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const pattern = /cat/;

console.log(pattern.test("The cat sat"));  // → true
console.log(pattern.test("The dog sat"));  // → false

The constructor form takes a string. It is useful when the pattern is built at run time.

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const word = "cat";
const pattern = new RegExp(word);

console.log(pattern.test("concatenate")); // → true

In the constructor form, backslashes in the string must be doubled: new RegExp("\\d+") is the same as /\d+/.

Your first useful pattern

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const hasDigit = /\d/;

console.log(hasDigit.test("abc123")); // → true
console.log(hasDigit.test("abcdef")); // → false

\d means “any digit”. The next lessons build up the vocabulary: Patterns and Character Classes, Quantifiers and Groups and Flags.

A mini example: highlight digits

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const text = "Order 42 shipped on day 7";
console.log(text.replace(/\d+/g, "#")); // → Order # shipped on day #

Do not fear them

Regex looks cryptic, but you only need a small part of it for most jobs. Learn it in small steps, test each pattern with a few examples (including ones that should not match), and write a short comment above any pattern that is not obvious.