Introduction to JavaScript regex lookahead

In regular expressions, a lookahead allows you to match X but only if it is followed by Y.

Here’s the syntax of the lookahead:

Code language: JavaScript (javascript)

In this syntax, the regex engine searches for X and only matches if it is followed by Y.

For example, suppose you have the following string:

Code language: JavaScript (javascript)

And you want to match the number 15 followed by a space and the literal string feet, not the number 1. To do that, you can use a lookahead in a regular expression:

Code language: JavaScript (javascript)

In this regular expression:

• \d+ matches one or more digits
• ?= is the lookahead syntax
• \s* matches zero or more whitespaces
• feet matches the literal string feet

The following code uses the above pattern to match the number that is followed by zero or more spaces and the literal string feet:

Code language: JavaScript (javascript)

Output:

Code language: PHP (php)

Regex multiple lookaheads

It’s possible to have multiple lookaheads in a regular expression with the following syntax:

Code language: JavaScript (javascript)

In this syntax, the regex engine performs the following steps:

1. Search for X
2. Check if Y is immediately after X, skip if it isn’t.
3. Check if Z is also immediately after Y; skip if it isn’t.
4. If both tests pass, return X as a match; otherwise, search for the next match.

Therefore, the X(?=Y)(?=Z) matches X followed by Y and Z simultaneously.

Regex negative lookaheads

Suppose you want to match the number 1 but not the number 15 in the following string:

Code language: JavaScript (javascript)

To do that, you use a negative lookahead. Here’s the syntax of the negative lookahead:

Code language: JavaScript (javascript)

In this syntax, the regex engine matches X only if it is not followed by Y.

The following example illustrates how to use a negative lookahead:

Code language: JavaScript (javascript)

Output:

Code language: plaintext (plaintext)

Summary

• A regex lookahead X(?=Y) matches X only if it is followed by Y.
• A negative regex lookahead X(?!Y) matches X only if it is not followed by Y.