Disallow use of the RegExp constructor in favor of regular expression literals (prefer-regex-literals)

There are two ways to create a regular expression:

• Regular expression literals, e.g., /abc/u.
• The RegExp constructor function, e.g., new RegExp("abc", "u") or RegExp("abc", "u").

The constructor function is particularly useful when you want to dynamically generate the pattern, because it takes string arguments.

When using the constructor function with string literals, don't forget that the string escaping rules still apply. If you want to put a backslash in the pattern, you need to escape it in the string literal. Thus, the following are equivalent:

new RegExp("^\\d\\.$");

/^\d\.$/;

// matches "0.", "1.", "2." ... "9."

In the above example, the regular expression literal is easier to read and reason about. Also, it's a common mistake to omit the extra \ in the string literal, which would produce a completely different regular expression:

new RegExp("^\d\.$");

// equivalent to /^d.$/, matches "d1", "d2", "da", "db" ...

When a regular expression is known in advance, it is considered a best practice to avoid the string literal notation on top of the regular expression notation, and use regular expression literals instead of the constructor function.

Rule Details

This rule disallows the use of the RegExp constructor function with string literals as its arguments.

This rule also disallows the use of the RegExp constructor function with template literals without expressions and String.raw tagged template literals without expressions.

The rule does not disallow all use of the RegExp constructor. It should be still used for dynamically generated regular expressions.

Examples of incorrect code for this rule:

new RegExp("abc");

new RegExp("abc", "u");

RegExp("abc");

RegExp("abc", "u");

new RegExp("\\d\\d\\.\\d\\d\\.\\d\\d\\d\\d");

RegExp(`^\\d\\.$`);

new RegExp(String.raw`^\d\.$`);

Examples of correct code for this rule:

/abc/;

/abc/u;

/\d\d\.\d\d\.\d\d\d\d/;

/^\d\.$/;

// RegExp constructor is allowed for dynamically generated regular expressions

new RegExp(pattern);

RegExp("abc", flags);

new RegExp(prefix + "abc");

RegExp(`${prefix}abc`);

new RegExp(String.raw`^\d\. ${sufix}`);

Further Reading

• MDN: Regular Expressions
• MDN: RegExp Constructor

Version

This rule was introduced in ESLint 6.4.0.

Resources

• Rule source
• Documentation source