Modifier: (?ims-ims:...)

flags1 Optional

A string of flags to enable. Can contain any combination of i, m, and s.

flags2 Optional

A string of flags to disable. Can contain any combination of i, m, and s, but must not contain any flags included in flags1.

pattern

A pattern consisting of anything you may use in a regex literal, including a disjunction.

Some flags change the meanings of regex syntax elements:

• The i flag makes the regex case-insensitive by making all literal characters and character classes implicitly be lowercase.
• The m flag changes the behavior of input boundary assertions ^ and $ to match the start and end of each line, in addition to the start and end of the input string.
• The s flag changes the behavior of the wildcard . character to match any character, including line terminators.

Sometimes you may want these changes to only take effect in a specific part of a regex pattern. You can do so by wrapping that part in a modifier. For example:

/(?i:Hello) world/;

In this regex, the i flag is only enabled for the Hello part of the pattern. The world part is case-sensitive. Therefore, it matches Hello world, hello world, and HELLO world, but not HELLO WORLD. The following is equivalent, by enabling the i flag globally, and then disabling it for the world part:

/Hello (?-i:world)/i;

The flags1 and flags2 parameters can contain any combination of i, m, and s. However, the flags must all be unique between flags1 and flags2—you cannot enable or disable a flag twice, or enable a flag and then immediately disable it.

The flags1 and flags2 parameters are optional, but at least one must be non-empty. (?flags1-:pattern) is a modifier that only enables flags (equivalent to (?flags1:pattern)). (?-flags2:pattern) is a modifier that only disables flags. (?:pattern) is just a non-capturing group, and (?-:pattern) is a syntax error.

Other flags don't make sense in a modifier and are thus syntax errors if included:

• The g and y flags determine how multiple calls to exec() behave and affect matching behavior of the whole regex.
• The d flag enables additional information in the exec() result and affects matching behavior of the whole regex.
• The u and v flags change the behavior of the regex engine in a way that's too complex to be locally modified. They also have global effects on the regex, such as how the lastIndex is advanced.

The following regex defines a format for a multiline string. The first ^ represents the start of the whole input string, by being inside a (?-m:) modifier, while all other ^ characters represent the start of a line:

const pattern = /(?-m:^)---\n^title:.*^slug:.*^---/ms;

const input = `---
title: "Modifier: (?ims-ims:...)"
slug: Web/JavaScript/Reference/Regular_expressions/Modifier
---`;

pattern.test(input); // true

// Extra line break at the start of string
const input2 = `\n${input}`;

pattern.test(input2); // false

Imagine you are finding all variable declarations called foo or bar (because they are bad names). The word may appear in any case, but you know the keyword is always lowercase, so you can do this:

const pattern = /(?:var|let|const) (?i:foo|bar)\b/;

pattern.test("let foo;"); // true
pattern.test("const BAR = 1;"); // true
pattern.test("Let foo be a number"); // false

• Groups and backreferences guide
• Regular expressions
• Non-capturing group: (?:...)