SyntaxError: Invalid flags supplied to RegExp constructor 'x' (V8-based)SyntaxError: Invalid regular expression flags (V8-based)SyntaxError: invalid regular expression flag x (Firefox)SyntaxError: Invalid flags supplied to RegExp constructor. (Safari)SyntaxError: Invalid regular expression: invalid flags (Safari)

The regular expression contains invalid flags, or valid flags have been used more than once in the expression.

The valid (allowed) flags ared,g,i,m,s,u,v, andy. They are introduced in more detail inRegular expressions > Advanced searching with flags.

Theu andv flags are mutually exclusive, so they cannot be used together. You can read the references to understand what their behavior differences are.

In a regular expression literal, which consists of a pattern enclosed between slashes, the flags are defined after the second slash.Regular expression flags can be used separately or together in any order.This syntax shows how to declare the flags using the regular expression literal:

const re = /pattern/flags;

They can also be defined in the constructor function of theRegExp object (second parameter):

const re = new RegExp("pattern", "flags");

Here is an example showing use of only correct flags.

/foo/g;/foo/gims;/foo/uy;

Below is an example showing the use of some invalid flagsb,a andr:

/foo/bar;// SyntaxError: invalid regular expression flag "b"

The code below is incorrect, becauseW,e andb are not valid flags.

const obj = {  url: /docs/Web,};// SyntaxError: invalid regular expression flag "W"

An expression containing two slashes is interpreted as a regular expression literal.Most likely the intent was to create a string literal, using single or double quotes as shown below:

const obj = {  url: "/docs/Web",};

• Regular expressions guide