eslint/docs/rules/require-unicode-regexp.md

   1 # Enforce the use of `u` flag on RegExp (require-unicode-regexp)
   2
   3 RegExp `u` flag has two effects:
   4
   5 1. **Make the regular expression handling UTF-16 surrogate pairs correctly.**
   6
   7     Especially, character range syntax gets the correct behavior.
   8
   9     ```js
  10     /^[👍]$/.test("👍") //→ false
  11     /^[👍]$/u.test("👍") //→ true
  12     ```
  13
  14 2. **Make the regular expression throwing syntax errors early as disabling [Annex B extensions](https://www.ecma-international.org/ecma-262/6.0/#sec-regular-expressions-patterns).**
  15
  16     Because of historical reason, JavaScript regular expressions are tolerant of syntax errors. For example, `/\w{1, 2/` is a syntax error, but JavaScript doesn't throw the error. It matches strings such as `"a{1, 2"` instead. Such a recovering logic is defined in Annex B.
  17
  18     The `u` flag disables the recovering logic Annex B defined. As a result, you can find errors early. This is similar to [the strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode).
  19
  20 Therefore, the `u` flag lets us work better with regular expressions.
  21
  22 ## Rule Details
  23
  24 This rule aims to enforce the use of `u` flag on regular expressions.
  25
  26 Examples of **incorrect** code for this rule:
  27
  28 ```js
  29 /*eslint require-unicode-regexp: error */
  30
  31 const a = /aaa/
  32 const b = /bbb/gi
  33 const c = new RegExp("ccc")
  34 const d = new RegExp("ddd", "gi")
  35 ```
  36
  37 Examples of **correct** code for this rule:
  38
  39 ```js
  40 /*eslint require-unicode-regexp: error */
  41
  42 const a = /aaa/u
  43 const b = /bbb/giu
  44 const c = new RegExp("ccc", "u")
  45 const d = new RegExp("ddd", "giu")
  46
  47 // This rule ignores RegExp calls if the flags could not be evaluated to a static value.
  48 function f(flags) {
  49     return new RegExp("eee", flags)
  50 }
  51 ```
  52
  53 ## When Not To Use It
  54
  55 If you don't want to notify regular expressions with no `u` flag, then it's safe to disable this rule.