regexLookaroundAssertions

Reports capturing groups at pattern boundaries that can be replaced with lookaround assertions.

✅ This rule is included in the ts stylisticStrict presets.

When a capturing group is at the start or end of a regex pattern and is only used to re-insert matched text in the replacement string (e.g., $1), it can be replaced with a lookaround assertion.

A capturing group at the start like /(prefix)rest/ can become a lookbehind: /(?<=prefix)rest/
A capturing group at the end like /rest(suffix)/ can become a lookahead: /rest(?=suffix)/

Doing so simplifies the replacement string since the matched text stays in place automatically. It can also make the regular expression easier to read and easier for engines to optimize for faster performance.

const 
const result: string
result = "JavaScript".
String.replace(searchValue: {
    [Symbol.replace](string: string, replaceValue: string): string;
}, replaceValue: string): string (+3 overloads)
Passes a string and
{@linkcode
replaceValue
}
to the [Symbol.replace] method on
{@linkcode
searchValue
}
. This method is expected to implement its own replacement algorithm.
@param ― searchValue An object that supports searching for and replacing matches within a string.
@param ― replaceValue The replacement text.
replace(/(Java)Script/, "$1");

const 
const result: string
result = "JavaScript".
String.replace(searchValue: {
    [Symbol.replace](string: string, replaceValue: string): string;
}, replaceValue: string): string (+3 overloads)
Passes a string and
{@linkcode
replaceValue
}
to the [Symbol.replace] method on
{@linkcode
searchValue
}
. This method is expected to implement its own replacement algorithm.
@param ― searchValue An object that supports searching for and replacing matches within a string.
@param ― replaceValue The replacement text.
replace(/Java(Script)/, "Type$1");

const 
const result: string
result = "prefix-text-suffix".
String.replace(searchValue: {
    [Symbol.replace](string: string, replaceValue: string): string;
}, replaceValue: string): string (+3 overloads)
Passes a string and
{@linkcode
replaceValue
}
to the [Symbol.replace] method on
{@linkcode
searchValue
}
. This method is expected to implement its own replacement algorithm.
@param ― searchValue An object that supports searching for and replacing matches within a string.
@param ― replaceValue The replacement text.
replace(
  /(prefix-)text(-suffix)/,
  "$1new$2",
);

const 
const result: string
result = "JavaScript".
String.replace(searchValue: {
    [Symbol.replace](string: string, replaceValue: string): string;
}, replaceValue: string): string (+3 overloads)
Passes a string and
{@linkcode
replaceValue
}
to the [Symbol.replace] method on
{@linkcode
searchValue
}
. This method is expected to implement its own replacement algorithm.
@param ― searchValue An object that supports searching for and replacing matches within a string.
@param ― replaceValue The replacement text.
replace(/(?<=Java)Script/, "");

const 
const result: string
result = "JavaScript".
String.replace(searchValue: {
    [Symbol.replace](string: string, replaceValue: string): string;
}, replaceValue: string): string (+3 overloads)
Passes a string and
{@linkcode
replaceValue
}
to the [Symbol.replace] method on
{@linkcode
searchValue
}
. This method is expected to implement its own replacement algorithm.
@param ― searchValue An object that supports searching for and replacing matches within a string.
@param ― replaceValue The replacement text.
replace(/Java(?=Script)/, "Type");

const 
const result: string
result = "prefix-text-suffix".
String.replace(searchValue: {
    [Symbol.replace](string: string, replaceValue: string): string;
}, replaceValue: string): string (+3 overloads)
Passes a string and
{@linkcode
replaceValue
}
to the [Symbol.replace] method on
{@linkcode
searchValue
}
. This method is expected to implement its own replacement algorithm.
@param ― searchValue An object that supports searching for and replacing matches within a string.
@param ― replaceValue The replacement text.
replace(
  /(?<=prefix-)text(?=-suffix)/,
  "new",
);

// Groups not at boundaries are fine
const 
const result: string
result = "abc".
String.replace(searchValue: {
    [Symbol.replace](string: string, replaceValue: string): string;
}, replaceValue: string): string (+3 overloads)
Passes a string and
{@linkcode
replaceValue
}
to the [Symbol.replace] method on
{@linkcode
searchValue
}
. This method is expected to implement its own replacement algorithm.
@param ― searchValue An object that supports searching for and replacing matches within a string.
@param ― replaceValue The replacement text.
replace(/a(b)c/, "$1");

Options

This rule is not configurable.

When Not To Use It

If you prefer explicit capturing groups for clarity, or if your team is less familiar with lookaround syntax, you might prefer to disable this rule. Some regex engines in other languages have limited or no lookaround support, so if you’re sharing patterns across environments, indexed replacements may be more portable.

Equivalents in Other Linters

ESLint: regexp/prefer-lookaround

Made with ❤️‍🔥 around the world by the Flint team and contributors.

regexLookaroundAssertions

Examples

Options

When Not To Use It

Further Reading

Equivalents in Other Linters