A regular expression literal can be confused with '/='


This warning was introduced in the original version of JSLint and existed in the same form in JSHint until version 1.0.0 when it was removed. ESLint has always issued the same warning.

When do I get this error?

The "A regular expression literal can be confused with '/='" error is thrown when JSLint, JSHint (prior to version 1.0.0) or ESLint encounters a regular expression literal that begins with the = character. In the following example we attempt to assign a regular expression literal to match the string "=1" to the variable x:

var regex = /=1/;

Why do I get this error?

This error is raised to highlight a potentially confusing piece of code. Your code will run fine if you do not fix this error, but it may be confusing to others, especially at first glance to someone quickly searching through your script.

The / character is ambiguous in JavaScript. It can either signify the start or end of a regular expression literal, as it does in the example above, or it can be interpreted as the division operator. Like most of the arithmetic operators, the division operator can be combined with the assignment operator to produce a shorthand:

var x = 10;
x /= 5; // Shorthand division-assignment operator

This ambiguity is not a problem because the parser should always be able to differentiate between the two uses. However, you can see why the regular expression example at the top of this page could cause initial confusion.

To solve this issue, you can simply escape the = character in the regular expression. This will behave in exactly the same way but since the = character is no longer the first, the error is not raised:

var regex = /\=1/;

Alternatively, you can use the RegExp constructor, which removes the need for the ambiguous delimiting / characters:

var regex = new RegExp("=1");

In ESLint the rule that generates this warning is named no-div-regex. You can disable it by setting it to 0, or enable it by setting it to 1.

About the author

James Allardice

This article was written by James Allardice, Software engineer at Tesco and orangejellyfish in London. Passionate about React, Node and writing clean and maintainable JavaScript. Uses linters (currently ESLint) every day to help achieve this.