JavaScript Regex Lookbehind

Summary: in this tutorial, you’ll learn how to use JavaScript regex lookbehind in regular expressions to match X if it is preceded by Y.

Introduction to the JavaScript regex lookbehind

In regular expressions, a lookbehind matches an element if there is another specific element before it. A lookbehind has the following syntax:

(?<=Y)X

In this syntax, the pattern match X if there is Y before it.

For example, suppose you want to match the number 900 not the number 1 in the following string:

'1 computer costs $900'Code language: JavaScript (javascript)

To do it, you use a lookahead in the regular expression as follows:

/(?<=\$)\d+/

In this regular expression:

• The (?<=\$) matches an element if there is a literal string $ before it. Because $ is a special character in the regex, we need to use the backslash \ to escape it. By doing this, the regex engine treats \$ as a literal character $.
• The \d+ matches one or more digits.

The following example illustrates how to use a lookbehind in a regular expression to match a number that has the $ sign before it:

const s = '1 computer costs $900';
const pattern = /(?<=\$)\d+/;

const match = s.match(pattern);
console.log(match);Code language: JavaScript (javascript)

Output:

[ '900', index: 18, input: '1 computer costs $900', groups: undefined ]Code language: JSON / JSON with Comments (json)

Negative lookbehind

To negate a lookbehind, you use a negative lookbehind with the following syntax:

(?<!Y)X

In this syntax, the regex engine matches X if there is no Y before it. The following example uses a regular expression with a negative lookbehind to match a number that doesn’t have the $ letter before it:

const s = '1 computer costs $900';
const pattern = /(?<!\$)\d+/;

const match = s.match(pattern);
console.log(match);Code language: JavaScript (javascript)

Output:

1

Summary

• A lookbehind (?<!Y)X matches X only if is is preceded by Y.
• A negative lookbehind (?<!Y)X matches X only if it is not preceded by Y.