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

   1 ---
   2 title: require-unicode-regexp
   3 rule_type: suggestion
   4 ---
   5
   6
   7 RegExp `u` flag has two effects:
   8
   9 1. **Make the regular expression handling UTF-16 surrogate pairs correctly.**
  10
  11     Especially, character range syntax gets the correct behavior.
  12
  13     ```js
  14     /^[👍]$/.test("👍") //→ false
  15     /^[👍]$/u.test("👍") //→ true
  16     ```
  17
  18 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).**
  19
  20     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.
  21
  22     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).
  23
  24 Therefore, the `u` flag lets us work better with regular expressions.
  25
  26 ## Rule Details
  27
  28 This rule aims to enforce the use of `u` flag on regular expressions.
  29
  30 Examples of **incorrect** code for this rule:
  31
  32 ::: incorrect
  33
  34 ```js
  35 /*eslint require-unicode-regexp: error */
  36
  37 const a = /aaa/
  38 const b = /bbb/gi
  39 const c = new RegExp("ccc")
  40 const d = new RegExp("ddd", "gi")
  41 ```
  42
  43 :::
  44
  45 Examples of **correct** code for this rule:
  46
  47 ::: correct
  48
  49 ```js
  50 /*eslint require-unicode-regexp: error */
  51
  52 const a = /aaa/u
  53 const b = /bbb/giu
  54 const c = new RegExp("ccc", "u")
  55 const d = new RegExp("ddd", "giu")
  56
  57 // This rule ignores RegExp calls if the flags could not be evaluated to a static value.
  58 function f(flags) {
  59     return new RegExp("eee", flags)
  60 }
  61 ```
  62
  63 :::
  64
  65 ## When Not To Use It
  66
  67 If you don't want to notify regular expressions with no `u` flag, then it's safe to disable this rule.