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46 Regular expressions (RegExp)

Icon “reading” Availability of features

Unless stated otherwise, each regular expression feature has been available since ES3.

46.1 Creating regular expressions

46.1.1 Literal vs. constructor

The two main ways of creating regular expressions are:

Both regular expressions have the same two parts:

Icon “tip” Recommended: flag /v ES2024

Flag /v enables important features and is recommended for all regular expressions.

46.1.2 Tip: Use String.raw literals with new RegExp()

If we use a normal string literal as an argument for new RegExp(), every regular expression backslash has to be escaped. If we use String.raw as a template tag then that’s not necessary. The following three regular expressions are all equivalent:

> /^\*$/.test('*')
true
> new RegExp('^\\*$', 'v').test('*')
true
> new RegExp(String.raw`^\*$`, 'v').test('*')
true

If we use String.raw often, we can abbreviate it:

// “Import” from namespace `String`
const {raw} = String;

const regExp = new RegExp(raw`^\*$`, 'v');

46.1.3 Cloning and non-destructively modifying regular expressions

There are two variants of the constructor RegExp():

The second variant is useful for cloning regular expressions, optionally while modifying them. Flags are immutable and this is the only way of changing them – for example:

function copyAndAddFlags(regExp, flagsToAdd='') {
  // The constructor doesn’t allow duplicate flags;
  // make sure there aren’t any:
  const newFlags = Array.from(
    new Set(regExp.flags + flagsToAdd)
  ).join('');
  return new RegExp(regExp, newFlags);
}
assert.equal(/abc/i.flags, 'i');
assert.equal(copyAndAddFlags(/abc/i, 'g').flags, 'gi');

new Set() iterates over its argument which means that a string is split into code points. Those code points become Set elements, which eliminates duplicates. Before we can join them into a string, we have to convert them to an Array and do so via Array.from().

46.2 Syntax characters and escaping

46.2.1 Syntax characters

At the top level of a regular expression, the following Syntax Characters are special. They are escaped by prefixing a backslash (\).

^ $ \ . * + ? ( ) [ ] { } |

This is an example:

> /\*/v.test('*')
true

In regular expression literals, we must escape slashes:

> /\//v.test('/')
true

In the argument of new RegExp(), we don’t have to escape slashes:

> new RegExp('/', 'v').test('/')
true

46.2.2 Illegal top-level escaping

With a Unicode flag (/u or /v), escaping a non-syntax character at the top level is a syntax error. That enables syntax such as \p{···}.

assert.throws(
  () => eval(String.raw`/\a/v`),
  {
    name: 'SyntaxError',
    message: 'Invalid regular expression: /\\a/v: Invalid escape',
  }
);
assert.throws(
  () => eval(String.raw`/\-/v`),
  {
    name: 'SyntaxError',
    message: 'Invalid regular expression: /\\-/v: Invalid escape',
  }
);

Without flag /u and /v, an escaped non-syntax character at the top level matches itself:

> /\a/.test('a')
true

46.2.3 Escaping inside character classes ([···])

The rules for escaping inside character classes are different if we use the recommended flag /v. We look at those first before we look at the rules for regular expressions that have no Unicode flag (neither /u nor /v).

46.2.3.1 Escaping inside character classes: flag /v

The following characters can be escaped via backslash:

Interestingly, we don’t always have to escape these characters. Only the following sequences of characters don’t match themselves and have to be escaped:

46.2.3.2 Escaping inside character classes: no Unicode flag (neither /u nor /v)

46.3 Regular expressions: the concept of a character

In the context of regular expressions, “character” means “atomic unit of text”:

For example, a dot (.) matches a single character:

> '🙂'.match(/./g) // code point with 2 code units
[ '\uD83D', '\uDE42' ]
> '🙂'.match(/./gv)
[ '🙂' ]

46.4 Syntax: matching single characters

These constructs match single characters:

46.5 Syntax: character class escapes

Character class escapes match sets of code units, sets of code points or sets of code point sequences.

46.5.1 Basic character class escapes (sets of code units): \d \D \s \S \w \W

The following character class escapes and their complements are always supported:

EscapeEquivalentComplement
Digits\d[0-9]\D
“Word” characters\w[a-zA-Z0-9_]\W
Whitespace\s\S

Note:

Examples:

> 'a7x4'.match(/\d/g)
[ '7', '4' ]
> 'a7x4'.match(/\D/g)
[ 'a', 'x' ]
> 'high - low'.match(/\w+/g)
[ 'high', 'low' ]
> 'hello\t\n everyone'.replaceAll(/\s/g, '-')
'hello---everyone'

46.5.2 Unicode property escapes: \p{} and \P{}

Unicode property escapes look like this:

There are two kinds of Unicode properties:

A quick example before we look at everything in more detail. This is how we can escape the Unicode character property White_Space:

// Match all code points that are whitespace
assert.deepEqual(
  'a\tb c'.match(/\p{White_Space}/gv),
  ['\t', ' ']
);

// Match all code points that are not whitespace
assert.deepEqual(
  'a\tb c'.match(/\P{White_Space}/gv),
  ['a', 'b', 'c']
);

46.5.3 Unicode character property escapes ES2018

With flag /u or flag /v, we can use \p{} and \P{} to specify sets of code points via Unicode character properties (we’ll learn more about those in the next subsection). That looks like this:

  1. \p{prop=value}: matches all characters whose Unicode character property prop has the value value.
  2. \P{prop=value}: matches all characters that do not have a Unicode character property prop whose value is value.
  3. \p{bin_prop}: matches all characters whose binary Unicode character property bin_prop is True.
  4. \P{bin_prop}: matches all characters whose binary Unicode character property bin_prop is False.

Comments:

Examples:

46.5.3.1 Unicode character properties

In the Unicode standard, each character has properties – metadata describing it. Properties play an important role in defining the nature of a character. Quoting the Unicode Standard, Sect. 3.3, D3:

The semantics of a character are determined by its identity, normative properties, and behavior.

These are a few examples of properties:

Further reading:

46.5.4 Unicode string property escapes ES2024

Only flag /v lets us use \p{} to specify sets of code point sequences via Unicode string properties (negation via \P{} is not supported). For example, RGI_Emoji is a Unicode string property:

> /^\p{RGI_Emoji}$/v.test('⛔') // 1 code point (1 code unit)
true
> /^\p{RGI_Emoji}$/v.test('🙂') // 1 code point (2 code units)
true
> /^\p{RGI_Emoji}$/v.test('😵‍💫') // 3 code points
true

Let’s see how the Unicode character property Emoji does with these inputs:

> /^\p{Emoji}$/v.test('⛔') // 1 code point (1 code unit)
true
> /^\p{Emoji}$/v.test('🙂') // 1 code point (2 code units)
true
> /^\p{Emoji}$/v.test('😵‍💫') // 3 code points
false

As expected, it only matches single code points.

46.5.4.1 Unicode string properties

For now, only the following Unicode string properties are supported by JavaScript:

The semantics of Unicode string properties are defined in text files that enumerate code point sequences likes this (\x{23} is #):

0023 FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: \x{23}
002A FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: *
0030 FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: 0
0031 FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: 1
0032 FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: 2
0033 FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: 3
0034 FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: 4
0035 FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: 5
0036 FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: 6
0037 FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: 7
0038 FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: 8
0039 FE0F 20E3 ; Emoji_Keycap_Sequence ; keycap: 9

Further reading:

46.6 Syntax: character classes

A character class wraps class ranges in square brackets. The class ranges specify a set of characters:

Rules for class ranges:

46.6.1 Code point sequences in character classes ES2024

With flag /v, we can use \q{} to add code point sequences to the set defined by a character class. This syntax is also called:

This is an example of using \q{}:

> /^[\q{😵‍💫}]$/v.test('😵‍💫')
true

Without \q{}, a grapheme cluster with multiple code points is still treated as multiple characters:

> /^[😵‍💫]$/v.test('😵‍💫')
false
> /^[\u{1F635}\u{200D}\u{1F4AB}]$/v.test('😵‍💫') // equivalent
false
> /^[😵‍💫]$/v.test('\u{1F635}')
true

We can use a single \q{} to add multiple code point sequences – if we separate them with pipes:

> /^[\q{abc|def}]$/v.test('abc')
true
> /^[\q{abc|def}]$/v.test('def')
true

46.6.2 Set operations for character classes ES2024

Flag /v enables set operations for character classes.

46.6.2.1 Nesting character classes

To enable set operations for character classes, we must be able to nest them. Character class escapes already provide some kind of nesting:

> /^[\d\w]$/v.test('7')
true
> /^[\d\w]$/v.test('H')
true
> /^[\d\w]$/v.test('?')
false

With flag /v, we can additionally nest character classes (the regular expression below is equivalent to the regular expression in the previous example):

> /^[[0-9][A-Za-z0-9_]]$/v.test('7')
true
> /^[[0-9][A-Za-z0-9_]]$/v.test('H')
true
> /^[[0-9][A-Za-z0-9_]]$/v.test('?')
false
46.6.2.2 Subtraction of character sets via --

We can use the -- operator to set-theoretically subtract the character sets defined by character classes or character class escapes:

> /^[\w--[a-g]]$/v.test('a')
false
> /^[\w--[a-g]]$/v.test('h')
true

> /^[\p{Number}--[0-9]]$/v.test('٣')
true
> /^[\p{Number}--[0-9]]$/v.test('3')
false

> /^[\p{RGI_Emoji}--\q{😵‍💫}]$/v.test('😵‍💫') // emoji has 3 code points
false
> /^[\p{RGI_Emoji}--\q{😵‍💫}]$/v.test('🙂')
true

Single code points can also be used on either side of the -- operator:

> /^[\w--a]$/v.test('a')
false
> /^[\w--a]$/v.test('b')
true
46.6.2.3 Intersection of character sets via &&

We can use the && operator to set-theoretically intersect the character sets defined by character classes or character class escapes:

> /[\p{ASCII}&&\p{Letter}]/v.test('D')
true
> /[\p{ASCII}&&\p{Letter}]/v.test('Δ')
false

> /^[\p{Script=Arabic}&&\p{Number}]$/v.test('٣')
true
> /^[\p{Script=Arabic}&&\p{Number}]$/v.test('ج')
false
46.6.2.4 Union of characters sets

Two compute the set-theoretical union of character sets, we only need to write their definining constructs next to each other inside a character class:

> /^[\p{Emoji_Keycap_Sequence}[a-z]]+$/v.test('a2️⃣c')
true

46.7 Syntax: capture groups

46.8 Syntax: quantifiers

By default, all of the following quantifiers are greedy (they match as many characters as possible):

To make them reluctant (so that they match as few characters as possible), put question marks (?) after them:

> /X.*X/.exec('XabcXdefX')[0]  // greedy
'XabcXdefX'
> /X.*?X/.exec('XabcXdefX')[0] // reluctant
'XabcX'

46.9 Syntax: assertions

Overview of available lookaround assertions:

PatternName
(?=«pattern»)Positive lookaheadES3
(?!«pattern»)Negative lookaheadES3
(?<=«pattern»)Positive lookbehindES2018
(?<!«pattern»)Negative lookbehindES2018

46.9.1 Lookahead assertions

Positive lookahead: (?=«pattern») matches if pattern matches what comes next.

Example: sequences of lowercase letters that are followed by an X. 

> 'abcX def'.match(/[a-z]+(?=X)/g)
[ 'abc' ]

Note that the X itself is not part of the matched substring.

Negative lookahead: (?!«pattern») matches if pattern does not match what comes next.

Example: sequences of lowercase letters that are not followed by an X.

> 'abcX def'.match(/[a-z]+(?!X)/g)
[ 'ab', 'def' ]

46.9.2 Lookbehind assertions ES2018

Positive lookbehind: (?<=«pattern») matches if pattern matches what came before.

Example: sequences of lowercase letters that are preceded by an X.

> 'Xabc def'.match(/(?<=X)[a-z]+/g)
[ 'abc' ]

Negative lookbehind: (?<!«pattern») matches if pattern does not match what came before.

Example: sequences of lowercase letters that are not preceded by an X.

> 'Xabc def'.match(/(?<!X)[a-z]+/g)
[ 'bc', 'def' ]

Example: replace “.js” with “.html”, but not in “Node.js”.

> 'Node.js: index.js and main.js'.replace(/(?<!Node)\.js/g, '.html')
'Node.js: index.html and main.html'

46.10 Syntax: disjunction (|)

Caveat: this operator has low precedence (binds very weakly). Use groups if necessary:

46.11 Regular expression flags

Literal flagProperty nameESDescription
dhasIndicesES2022Switch on match indices
gglobalES3Match multiple times
iignoreCaseES3Match case-insensitively
mmultilineES3^ and $ match per line
sdotAllES2018Dot matches line terminators
uunicodeES6Unicode mode
vunicodeSetsES2024Unicode sets mode (recommended)
ystickyES6No characters between matches

Table 46.1: These are the regular expression flags supported by JavaScript.

The following regular expression flags are available in JavaScript (table 46.1 provides a compact overview):

46.11.1 How to order regular expression flags?

Consider the following regular expression: /“([^”]+)”/vdg

In which order should we list its flags? Two options are:

  1. Alphabetical order: /dgv
  2. In order of importance (arguably, /v is most fundamental etc.): /vgd

Given that (2) is not obvious, (1) is the better choice. JavaScript also uses it for the RegExp property .flags :

> /-/gymdivs.flags
'dgimsvy'

46.11.2 Without the Unicode flags /u and /v: characters are UTF-16 code units

Without the Unicode flags /u and /v, most constructs work with single UTF-16 code units, which is problematic whenever a character has more than one code unit – e.g. 🙂:

> '🙂'.length
2

Without Unicode flags, the largest character we can escape is a code unit, via a code unit escape. It consists of \u followed by four hexadecimal digits:

> /^\uD83D\uDE42$/.test('🙂')
true

The dot operator (.) matches code units, which is why we get two matches instead of one:

> '🙂'.match(/./g)
[ '\uD83D', '\uDE42' ]

Quantifiers apply to code units and therefore only repeat the second half of 🙂:

> /^🙂{2}$/.test('\uD83D\uDE42\uDE42')
true
> /^\uD83D\uDE42{2}$/.test('\uD83D\uDE42\uDE42') // equivalent
true

Character class escapes define sets of code units. Therefore, the class escape \D for “not a decimal digit” gets two matches:

> '🙂'.match(/\D/g)
[ '\uD83D', '\uDE42' ]

Character classes define sets of code units. Therefore, putting 🙂 in a character class has unintuitive consequences:

> /^[🙂]$/.test('🙂')
false
> /^[\uD83D\uDE42]$/.test('\uD83D\uDE42') // equivalent
false
> /^[🙂]$/.test('\uD83D')
true

46.11.3 Flag /u: characters are code points ES6

In the previous subsection, we encountered problems when we wanted to match a code point with more than one UTF-16 code unit – such as 🙂. Flag /u enables support for code points and fixes those problems.

We can escape code points via code point escapes – \u{} with one to six hexadecimal digits:

> /^\u{1F642}$/u.test('🙂')
true

The dot operator (.) matches code points:

> '🙂'.match(/./gu)
[ '🙂' ]

Quantifiers apply to code points:

> /^🙂{2}$/u.test('🙂🙂')
true

Character class escapes define sets of code points:

> '🙂'.match(/\D/gu)
[ '🙂' ]

A new kind of character class escapes is supported – Unicode character property escapes specify sets of code points:

> /^\p{Emoji}$/u.test('⛔') // 1 code point (1 code unit)
true
> /^\p{Emoji}$/u.test('🙂') // 1 code point (2 code units)
true

Character classes also define sets of code points:

> /^[🙂]$/u.test('🙂')
true
> /^[🙂]$/u.test('\uD83D')
false

46.11.4 Flag /v: limited support for multi-code-point grapheme clusters ES2024

Icon “tip” Use flag /v whenever you can

This flag improves many aspects of JavaScript’s regular expressions and should be used by default. If a platform doesn’t support it yet, you should at least use /u.

46.11.4.1 Limitation of flag /u: handling grapheme clusters with more than one code point

Some font glyphs are represented by grapheme clusters (code point sequences) with more than one code point – e.g. 😵‍💫:

> Array.from('😵‍💫').length // count code points
3

Flag /u does not help us with those kinds of grapheme clusters:

// Grapheme cluster is not matched by single dot
assert.equal(
  '😵‍💫'.match(/./gu).length, 3
);

// Quantifiers only repeat last code point of grapheme cluster
assert.equal(
  /^😵‍💫{2}$/u.test('😵‍💫😵‍💫'), false
);

// Character class escapes only match single code points
assert.equal(
  /^\p{Emoji}$/u.test('😵‍💫'), false
);

// Character classes only match single code points
assert.equal(
  /^[😵‍💫]$/u.test('😵‍💫'), false
);
46.11.4.2 Flag /v: Unicode string property escapes and character class string literals

Flag /v works like flag /u but provides better support for multi-code-point grapheme clusters. It doesn’t switch from code points to grapheme clusters everywhere, but it does fix the last two issues we encountered in the previous subsection – by adding support for multi-code-point grapheme clusters to:

46.11.4.3 Flag /v: character class set operations

Character classes can be nested and combined via the set operations subtraction and intersection – see “Set operations for character classes ES2024” (§46.6.2).

46.11.4.4 Flag /v: improved case-insensitive matching

Flag /u has a quirk when it comes to case-insensitive matching: Using \P{···} produces different results than [^\p{···}]:

> /^\P{Lowercase_Letter}$/iu.test('A')
true
> /^\P{Lowercase_Letter}$/iu.test('a')
true

> /^[^\p{Lowercase_Letter}]$/iu.test('A')
false
> /^[^\p{Lowercase_Letter}]$/iu.test('a')
false

Observations:

Flag /v fixes that quirk:

> /^\P{Lowercase_Letter}$/iv.test('A')
false
> /^\P{Lowercase_Letter}$/iv.test('a')
false

> /^[^\p{Lowercase_Letter}]$/iv.test('A')
false
> /^[^\p{Lowercase_Letter}]$/iv.test('a')
false

Further reading:

46.12 Pattern modifiers (inline flags) ES2025

Pattern modifiers let us apply a flag to a part of a regular expression (vs. all of the regular expression) – for example, in the following regular expression, the flag i is only applied to “HELLO”:

> /^x(?i:HELLO)x$/.test('xHELLOx')
true
> /^x(?i:HELLO)x$/.test('xhellox')
true
> /^x(?i:HELLO)x$/.test('XhelloX')
false

46.12.1 The syntax of pattern modifiers

This is what the syntax looks like:

(?ims-ims:pattern)
(?ims:pattern)
(?-ims:pattern)

Notes:

Let’s change the previous example: Now all of the regular expression is case-insensitive – except for “HELLO”:

> /^x(?-i:HELLO)x$/i.test('xHELLOx')
true
> /^x(?-i:HELLO)x$/i.test('XHELLOX')
true
> /^x(?-i:HELLO)x$/i.test('XhelloX')
false

46.12.2 Which flags are supported?

The following flags can be used in pattern modifiers:

Literal flagProperty nameESDescription
iignoreCaseES3Match case-insensitively
mmultilineES3^ and $ match per line
sdotAllES2018Dot matches line terminators

For more information, see “Regular expression flags” (§46.11).

The remaining flags are not supported because they would either make regular expression semantics too complicated (e.g. flag v) or because they only make sense if applied to the whole regular expression (e.g. flag g).

46.12.3 Use case: changing flags for part of a regular expression

It’s sometimes useful to change flags for part of a regular expression. For example, Ron Buckton explains that changing flag m helps with matching a Markdown frontmatter block at the start of a file (I slightly edited his version):

const re = /(?-m:^)---\r?\n((?:^(?!---$).*\r?\n)*)^---$/m;
assert.equal(re.test('---a'), false);
assert.equal(re.test('---\n---'), true);
assert.equal(
  re.exec('---\n---')[1],
  ''
);
assert.equal(
  re.exec('---\na: b\n---')[1],
  'a: b\n'
);

How does this regular expression work?

This is the regular expression, formatted with insignificant whitespace and explanatory comments:

(?-m:^)---\r?\n  # first line of string
(  # capturing group for the frontmatter
  (?:  # pattern for one line (non-capturing group)
    ^(?!---$)  # line must not start with "---" + EOL (lookahead)
    .*\r?\n
  )*
)
^---$  # closing delimiter of frontmatter

46.12.4 Use case: inlining flags

In some situations, flags being outside the actual regular expressions is inconvenient. Then pattern modifiers help. Examples include:

46.12.5 Use case: regular expression fragments that change flags

In complex applications, it helps if you can compose large regular expressions out of smaller regular expressions. The aforementioned Regex+ library supports that. If a smaller regular expression needs different flags (e.g. because it wants to ignore case) then it can – thanks to pattern modifiers.

46.13 Properties of regular expression objects

Noteworthy:

46.13.1 Flags as properties

Each regular expression flag exists as a property with a longer, more descriptive name:

> /a/i.ignoreCase
true
> /a/.ignoreCase
false

This is the complete list of flag properties:

46.13.2 Other properties

Each regular expression also has the following properties:

46.14 Match objects

Several regular expression-related methods return so-called match objects to provide detailed information for the locations where a regular expression matches an input string. These methods are:

This is an example:

assert.deepEqual(
  /(a+)b/d.exec('ab aaab'),
  {
    0: 'ab',
    1: 'a',
    index: 0,
    input: 'ab aaab',
    groups: undefined,
    indices: {
      0: [0, 2],
      1: [0, 1],
      groups: undefined
    },
  }
);

The result of .exec() is a match object for the first match with the following properties:

46.14.1 Match indices in match objects ES2022

Match indices are a feature of match objects: If we turn it on via the regular expression flag /d (property .hasIndices), they record the start and end indices of where groups were captured.

46.14.1.1 Match indices for numbered groups

This is how we access the captures of numbered groups:

const matchObj = /(a+)(b+)/d.exec('aaaabb');
assert.equal(
  matchObj[1], 'aaaa'
);
assert.equal(
  matchObj[2], 'bb'
);

Due to the regular expression flag /d, matchObj also has a property .indices that records for each numbered group where it was captured in the input string:

assert.deepEqual(
  matchObj.indices[1], [0, 4]
);
assert.deepEqual(
  matchObj.indices[2], [4, 6]
);
46.14.1.2 Match indices for named groups

The captures of named groups are accessed like this:

const matchObj = /(?<as>a+)(?<bs>b+)/d.exec('aaaabb');
assert.equal(
  matchObj.groups.as, 'aaaa'
);
assert.equal(
  matchObj.groups.bs, 'bb'
);

Their indices are stored in matchObj.indices.groups:

assert.deepEqual(
  matchObj.indices.groups.as, [0, 4]
);
assert.deepEqual(
  matchObj.indices.groups.bs, [4, 6]
);
46.14.1.3 A more realistic example

One important use case for match indices are parsers that point to where exactly a syntactic error is located. The following code solves a related problem: It points to where quoted content starts and where it ends (see demonstration at the end).

const reQuoted = /“([^”]+)”/dgv;
function pointToQuotedText(str) {
  const startIndices = new Set();
  const endIndices = new Set();
  for (const match of str.matchAll(reQuoted)) {
    const [start, end] = match.indices[1];
    startIndices.add(start);
    endIndices.add(end);
  }
  let result = '';
  for (let index=0; index < str.length; index++) {
    if (startIndices.has(index)) {
      result += '[';
    } else if (endIndices.has(index+1)) {
      result += ']';
    } else {
      result += ' ';
    }
  }
  return result;
}

assert.equal(
  pointToQuotedText(
    'They said “hello” and “goodbye”.'),
    '           [   ]       [     ]  '
);

46.15 Methods for working with regular expressions

46.15.1 By default, regular expressions match anywhere in a string

By default, regular expressions match anywhere in a string:

> /a/.test('__a__')
true

We can change that by using assertions such as ^ (or by using the flag /y):

> /^a/.test('__a__')
false
> /^a/.test('a__')
true

46.15.2 string.match(regExp) without /g: getting a match object for the first match

Without the flag /g, string.match(regExp) returns a match object for the first match of regExp in str:

assert.deepEqual(
  'ab aab'.match(/(a+)b/),
  {
    0: 'ab',
    1: 'a',
    index: 0,
    input: 'ab aab',
    groups: undefined,
  }
);
46.15.2.1 Named capture groups ES2018

The previous example contained a single numbered group. The following example demonstrates named groups:

assert.deepEqual(
  'ab aab'.match(/(?<as>a+)b/),
  {
    0: 'ab',
    1: 'a',
    index: 0,
    input: 'ab aab',
    groups: { as: 'a' },
  }
);

In the result of .match(), we can see that a named group is also a numbered group – its capture exists twice:

46.15.2.2 Duplicate named capture groups ES2025

Since ECMAScript 2025, we can use the same group name twice – as long as it appears in different alternatives:

const RE = /(?<a>a(?<xs>x+))|(?<b>b(?<xs>x+))/v;
assert.deepEqual(
  'axx'.match(RE).groups,
  {
    a: 'axx',
    xs: 'xx',
    b: undefined,
    __proto__: null,
  }
);
assert.deepEqual(
  'bxx'.match(RE).groups,
  {
    a: undefined,
    xs: 'xx',
    b: 'bxx',
    __proto__: null,
  }
);

The following is not allowed (group xs would match twice):

assert.throws(
  () => eval('/(?<a>a(?<xs>x+))(?<b>b(?<xs>x+))/v'),
  /^SyntaxError:.* Duplicate capture group name$/
);

46.15.3 string.match(regExp) with /g: getting all group 0 captures ES3

With flag /g, string.match(regExp) returns all substrings of str that match regExp:

> 'ab aab'.match(/(a+)b/g)
[ 'ab', 'aab' ]

If there is no match, .match() returns null:

> 'xyz'.match(/(a+)b/g)
null

We can use the nullish coalescing operator (??) to protect ourselves against null:

const numberOfMatches = (str.match(regExp) ?? []).length;

We can also use optional chaining (?.) and combine it with the nullish coalescing operator:

const numberOfMatches = str.match(regExp)?.length ?? 0;

46.15.4 string.matchAll(regExp): getting an iterable over all match objects ES2020

This is how .matchAll() is invoked:

const matchIterable = str.matchAll(regExp);

Given a string and a regular expression, .matchAll() returns an iterable over the match objects of all matches.

In the following example, we use Array.from() to convert iterables to Arrays so that we can compare them better.

> Array.from('-a-a-a'.matchAll(/-(a)/gv))
[
  { 0:'-a', 1:'a', index: 0, input: '-a-a-a', groups: undefined },
  { 0:'-a', 1:'a', index: 2, input: '-a-a-a', groups: undefined },
  { 0:'-a', 1:'a', index: 4, input: '-a-a-a', groups: undefined },
]

Flag /g must be set:

> Array.from('-a-a-a'.matchAll(/-(a)/v))
TypeError: String.prototype.matchAll called with a non-global
RegExp argument

.matchAll() isn’t affected by regExp.lastIndex and doesn’t change it.

Icon “exercise” Exercise: Extracting quoted text via .matchAll()

exercises/regexps/extract_quoted_test.mjs

46.15.4.1 Implementing .matchAll()

.matchAll() could be implemented via .exec() as follows:

function* matchAll(str, regExp) {
  if (!regExp.global) {
    throw new TypeError('Flag /g must be set!');
  }
  // Preserve and reset flags
  const localCopy = new RegExp(regExp, regExp.flags);
  let match;
  while (match = localCopy.exec(str)) {
    yield match;
  }
}

Making a local copy ensures two things:

Using matchAll():

const str = '"fee" "fi" "fo" "fum"';
const regex = /"([^"]*)"/g;

for (const match of matchAll(str, regex)) {
  console.log(match[1]);
}

Output:

fee
fi
fo
fum

46.15.5 regExp.exec(str): capturing groups ES3

46.15.5.1 regExp.exec(str) without /g: getting a match object for the first match

Without flag /g, regExp.exec() works like string.match() – it returns a single match object.

46.15.5.2 regExp.exec(str) with /g: looping over all matches

Icon “tip” Better alternative for retrieving all matches: string.matchAll(regExp) ES2020

Since ECMAScript 2020, JavaScript has another method for retrieving all matches: string.matchAll(regExp). That method is easier and safer to use: It returns an iterable, is unaffected by .lastIndex and throws an exception if flag /g is missing.

If we want to retrieve all matches of a regular expression (not just the first one), we need to switch on the flag /g. Then we can call .exec() multiple times and get one match each time. After the last match, .exec() returns null.

> const regExp = /(a+)b/g;
> regExp.exec('ab aab')
{ 0: 'ab', 1: 'a', index: 0, input: 'ab aab', groups: undefined }
> regExp.exec('ab aab')
{ 0: 'aab', 1: 'aa', index: 3, input: 'ab aab', groups: undefined }
> regExp.exec('ab aab')
null

Therefore, we can loop over all matches as follows:

const regExp = /(a+)b/g;
const str = 'ab aab';

let match;
// Check for null via truthiness
// Alternative: while ((match = regExp.exec(str)) !== null)
while (match = regExp.exec(str)) {
  console.log(match[1]);
}

Output:

a
aa

46.15.6 string.match() vs. string.matchAll() vs. regExp.exec()

The following table summarizes the differences between three methods:

Without /gWith /g
string.match(regExp)First match objectArray of group 0 captures
string.matchAll(regExp)TypeErrorIterable over match objects
regExp.exec(string)First match objectNext match object or null

46.15.7 Replacing with string.replace() and string.replaceAll()

Both replacing methods have two parameters:

searchValue can be:

replacementValue can be:

The two methods differ as follows:

This table summarizes how that works:

Search for: →stringRegExp w/o /gRegExp with /g
.replaceFirst occurrenceFirst occurrence(All occurrences)
.replaceAllAll occurrencesTypeErrorAll occurrences

The last column of .replace() is in parentheses because this method existed long before .replaceAll() and therefore supports functionality that should now be handled via the latter method. If we could change that, .replace() would throw a TypeError here.

We first explore how .replace() and .replaceAll() work individually when replacementValue is a simple string (without the character $). Then we examine how both are affected by more complicated replacement values.

46.15.7.1 string.replace(searchValue, replacementValue) ES3

How .replace() operates is influenced by its first parameter searchValue:

If we want to replace every occurrence of a string, we have two options:

46.15.7.2 string.replaceAll(searchValue, replacementValue) ES2021

How .replaceAll() operates is influenced by its first parameter searchValue:

46.15.7.3 The parameter replacementValue of .replace() and .replaceAll()

So far, we have only used the parameter replacementValue with simple strings, but it can do more. If its value is:

46.15.7.4 replacementValue is a string

If the replacement value is a string, the dollar sign has special meaning – it inserts text matched by the regular expression:

TextResult
$$single $
$&complete match
$`text before match
$'text after match
$ncapture of numbered group n (n > 0)
$<name>capture of named group name (ES2018)

Example: Inserting the text before, inside, and after the matched substring.

> 'a1 a2'.replaceAll(/a/g, "($`|$&|$')")
'(|a|1 a2)1 (a1 |a|2)2'

Example: Inserting the captures of numbered groups.

> const regExp = /^([A-Za-z]+): (.*)$/gv;
> 'first: Jane'.replaceAll(regExp, 'KEY: $1, VALUE: $2')
'KEY: first, VALUE: Jane'

Example: Inserting the captures of named groups.

> const regExp = /^(?<key>[A-Za-z]+): (?<value>.*)$/gv;
> 'first: Jane'.replaceAll(regExp, 'KEY: $<key>, VALUE: $<value>')
'KEY: first, VALUE: Jane'

Icon “exercise” Exercise: Change quotes via .replace() and a named group

exercises/regexps/change_quotes_test.mjs

46.15.7.5 replacementValue is a function

If the replacement value is a function, we can compute each replacement. In the following example, we multiply each non-negative integer that we find by two.

assert.equal(
  '3 cats and 4 dogs'.replaceAll(/[0-9]+/g, (all) => 2 * Number(all)),
  '6 cats and 8 dogs'
);

The replacement function gets the following parameters. Note how similar they are to match objects. These parameters are all positional, but I’ve included how one might name them:

If we are only interested in groups, we can use the following technique:

const result = 'first=jane, last=doe'.replace(
  /(?<key>[a-z]+)=(?<value>[a-z]+)/g,
  (...args) => { // (A)
    const groups = args.at(-1); // (B)
    const {key, value} = groups;
    return key.toUpperCase() + '=' + value.toUpperCase();
  });
assert.equal(result, 'FIRST=JANE, LAST=DOE');

Due to the rest parameter in line A, args contains an Array with all parameters. We access the last parameter via the Array method .at() in line B.

46.15.8 regExp.test(str): is there a match? ES3

The regular expression method regExp.test(str) returns true if regExp matches str:

> /bc/.test('ABCD')
false
> /bc/i.test('ABCD')
true
> /\.mjs$/.test('main.mjs')
true

With .test() we should normally avoid the /g flag. If we use it, we generally don’t get the same result every time we call the method:

> const r = /a/g;
> r.test('aab')
true
> r.test('aab')
true
> r.test('aab')
false

The results are due to /a/ having two matches in the string. After all of those were found, .test() returns false. For more information, see “The flags /g and /y, and the property .lastIndex (advanced)” (§46.16).

46.15.9 string.search(regExp): at what index is the match? ES3

The string method .search() returns the first index of str at which there is a match for regExp:

> '_abc_'.search(/abc/)
1
> 'main.mjs'.search(/\.mjs$/)
4

46.15.10 string.split(separator, limit?): splitting strings ES3

Splits the string into an Array of substrings – the strings that occur between the separators.

The separator can be a string:

> 'a : b : c'.split(':')
[ 'a ', ' b ', ' c' ]

It can also be a regular expression:

> 'a x:yyy b'.split(/x+:y+/)
[ 'a ', ' b' ]
> 'a x:yyy b'.split(/(x+):(y+)/)
[ 'a ', 'x', 'yyy', ' b' ]

The last invocation demonstrates that captures made by groups in the regular expression become elements of the returned Array.

If we want the separators to be part of the returned string fragments, we can use a regular expression with a lookbehind assertion or a lookahead assertion:

> 'a: b: c'.split(/(?<=:) */)
[ 'a:', 'b:', 'c' ]
> 'a :b :c'.split(/ *(?=:)/)
[ 'a', ':b', ':c' ]

Thanks to the lookaround assertions, the regular expression used for splitting leaves the colons in the fragments and only removes the spaces following or preceding the colons.

Pitfall: .split('') splits into JavaScript characters, but we usually want to split into grapheme clusters or at least Unicode code points. Therefore, it’s better to use Intl.Segmenter or Array.from() for splitting. For more information, see “Atoms of text: code points, JavaScript characters, grapheme clusters” (§22.7).

46.16 The flags /g and /y, and the property .lastIndex (advanced)

In this section, we examine how the RegExp flags /g and /y work and how they depend on the RegExp property .lastIndex. We’ll also discover an interesting use case for .lastIndex that you may find surprising.

46.16.1 The flags /g and /y

Every method reacts differently to /g and /y; this gives us a rough general idea:

If a regular expression has neither the flag /g nor the flag /y, matching happens once and starts at the beginning.

With either /g or /y, matching is performed relative to a “current position” inside the input string. That position is stored in the regular expression property .lastIndex.

There are three groups of regular-expression-related methods:

  1. The string methods .search(regExp) and .split(regExp) completely ignore /g and /y (and therefore also .lastIndex).

  2. The RegExp methods .exec(str) and .test(str) change in two ways if either /g or /y is set.

    First, we get multiple matches, by calling one method repeatedly. Each time, it returns either another result (a match object or true) or an “end of results” value (null or false).

    Second, the regular expression property .lastIndex is used to step through the input string. On one hand, .lastIndex determines where matching starts:

    • /g means that a match must begin at .lastIndex or later.

    • /y means that a match must begin at .lastIndex. That is, the beginning of the regular expression is anchored to .lastIndex.

      Note that ^ and $ continue to work as usually: They anchor matches to the beginning or end of the input string, unless .multiline is set. Then they anchor to the beginnings or ends of lines.

    On the other hand, .lastIndex is set to one plus the last index of the previous match.

  3. The remaining methods are:

    • The string methods .match(regExp) and .matchAll(regExp) (throws without /g)
    • The string methods .replace(regExp, str) and .replaceAll(regExp, str) (throws without /g)

    These are all affected as follows:

    • /g leads to multiple matches.
    • /y leads to a single match that must start at .lastIndex.
    • /gy leads to multiple matches without gaps.

This was a first overview. The next sections get into more details.

46.16.2 How exactly are methods affected by /g and /y?

The following methods are unaffected by /g and /y:

46.16.2.1 string.match(regExp) ES3

Without /g and /y, .match() ignores .lastIndex and always returns a match object for the first match.

> const re = /#/; re.lastIndex = 1;
> ['##-#'.match(re), re.lastIndex]
[{ 0: '#', index: 0, input: '##-#' }, 1]
> ['##-#'.match(re), re.lastIndex]
[{ 0: '#', index: 0, input: '##-#' }, 1]

With /y, the match must start at exactly .lastIndex. .lastIndex is updated. If there is no match, null is returned.

> const re = /#/y; re.lastIndex = 1;
> ['##-#'.match(re), re.lastIndex]
[{ 0: '#', index: 1, input: '##-#' }, 2]
> ['##-#'.match(re), re.lastIndex]
[null, 0]

With /g, we get all matches (group 0) in an Array. .lastIndex is ignored and reset to zero.

> const re = /#/g; re.lastIndex = 1;
> '##-#'.match(re)
['#', '#', '#']
> re.lastIndex
0

/gy works similarly to /g, but no gaps between matches are allowed:

> const re = /#/gy; re.lastIndex = 1;
> '##-#'.match(re)
['#', '#']
> re.lastIndex
0
46.16.2.2 string.matchAll(regExp) ES2020

If /g is not set, .matchAll() throws an exception:

> const re = /#/y; re.lastIndex = 1;
> '##-#'.matchAll(re)
TypeError: String.prototype.matchAll called with
a non-global RegExp argument

If /g is set, matching starts at .lastIndex and that property isn’t changed:

> const re = /#/g; re.lastIndex = 1;
> Array.from('##-#'.matchAll(re))
[
  { 0: '#', index: 1, input: '##-#' },
  { 0: '#', index: 3, input: '##-#' },
]
> re.lastIndex
1

/gy works similarly to /g, but no gaps between matches are allowed:

> const re = /#/gy; re.lastIndex = 1;
> Array.from('##-#'.matchAll(re))
[
  { 0: '#', index: 1, input: '##-#' },
]
> re.lastIndex
1
46.16.2.3 regExp.exec(str) ES3

Without /g and /y, .exec() ignores .lastIndex and always returns a match object for the first match:

> const re = /#/; re.lastIndex = 1;
> [re.exec('##-#'), re.lastIndex]
[{ 0: '#', index: 0, input: '##-#' }, 1]
> [re.exec('##-#'), re.lastIndex]
[{ 0: '#', index: 0, input: '##-#' }, 1]

With /g, the match must start at .lastIndex or later. .lastIndex is updated. If there is no match, null is returned.

> const re = /#/g; re.lastIndex = 1;
> [re.exec('##-#'), re.lastIndex]
[{ 0: '#', index: 1, input: '##-#' }, 2]
> [re.exec('##-#'), re.lastIndex]
[{ 0: '#', index: 3, input: '##-#' }, 4]
> [re.exec('##-#'), re.lastIndex]
[null, 0]

With /y, the match must start at exactly .lastIndex. .lastIndex is updated. If there is no match, null is returned.

> const re = /#/y; re.lastIndex = 1;
> [re.exec('##-#'), re.lastIndex]
[{ 0: '#', index: 1, input: '##-#' }, 2]
> [re.exec('##-#'), re.lastIndex]
[null, 0]

With /gy, .exec() behaves the same as with /y.

46.16.2.4 string.replace(regExp, str) ES3

Without /g and /y, only the first occurrence is replaced:

> const re = /#/; re.lastIndex = 1;
> '##-#'.replace(re, 'x')
'x#-#'
> re.lastIndex
1

With /g, all occurrences are replaced. .lastIndex is ignored but reset to zero.

> const re = /#/g; re.lastIndex = 1;
> '##-#'.replace(re, 'x')
'xx-x'
> re.lastIndex
0

With /y, only the (first) occurrence at .lastIndex is replaced. .lastIndex is updated.

> const re = /#/y; re.lastIndex = 1;
> '##-#'.replace(re, 'x')
'#x-#'
> re.lastIndex
2

/gy works like /g, but gaps between matches are not allowed:

> const re = /#/gy; re.lastIndex = 1;
> '##-#'.replace(re, 'x')
'xx-#'
> re.lastIndex
0
46.16.2.5 string.replaceAll(regExp, str) ES2021

.replaceAll() works like .replace() but throws an exception if /g is not set:

> const re = /#/y; re.lastIndex = 1;
> '##-#'.replaceAll(re, 'x')
TypeError: String.prototype.replaceAll called
with a non-global RegExp argument
46.16.2.6 regExp.test(str) ES3

This method behaves the same same as .exec(), but instead of returning a match object, it returns true, and instead of returning null, it returns false.

For example, without either /g or /y, the result is always true:

> const re = /#/; re.lastIndex = 1;
> [re.test('##-#'), re.lastIndex]
[true, 1]
> [re.test('##-#'), re.lastIndex]
[true, 1]

With /g, there are two matches:

> const re = /#/g; re.lastIndex = 1;
> [re.test('##-#'), re.lastIndex]
[true, 2]
> [re.test('##-#'), re.lastIndex]
[true, 4]
> [re.test('##-#'), re.lastIndex]
[false, 0]

With /y, there is only one match:

> const re = /#/y; re.lastIndex = 1;
> [re.test('##-#'), re.lastIndex]
[true, 2]
> [re.test('##-#'), re.lastIndex]
[false, 0]

With /gy, .test() behaves the same as with /y.

46.16.3 Four pitfalls of /g and /y and how to deal with them

We will first look at four pitfalls of /g and /y and then at ways of dealing with those pitfalls.

46.16.3.1 Pitfall 1: We can’t inline a regular expression with /g or /y

A regular expression with /g can’t be inlined. For example, in the following while loop, the regular expression is created fresh, every time the condition is checked. Therefore, its .lastIndex is always zero and the loop never terminates.

let matchObj;
// Infinite loop
while (matchObj = /a+/g.exec('bbbaabaaa')) {
  console.log(matchObj[0]);
}

With /y, the problem is the same.

46.16.3.2 Pitfall 2: Removing /g or /y can break code

If code expects a regular expression with /g and has a loop over the results of .exec() or .test(), then a regular expression without /g can cause an infinite loop:

function collectMatches(regExp, str) {
  const matches = [];
  let matchObj;
  // Infinite loop
  while (matchObj = regExp.exec(str)) {
    matches.push(matchObj[0]);
  }
  return matches;
}
collectMatches(/a+/, 'bbbaabaaa'); // Missing: flag /g

Why is there an infinite loop? Because .exec() always returns the first result, a match object, and never null.

With /y, the problem is the same.

46.16.3.3 Pitfall 3: Adding /g or /y can break code

With .test(), there is another caveat: It is affected by .lastIndex. Therefore, if we want to check exactly once if a regular expression matches a string, then the regular expression must not have /g. Otherwise, we generally get a different result every time we call .test():

> const regExp = /^X/g;
> [regExp.test('Xa'), regExp.lastIndex]
[ true, 1 ]
> [regExp.test('Xa'), regExp.lastIndex]
[ false, 0 ]
> [regExp.test('Xa'), regExp.lastIndex]
[ true, 1 ]

The first invocation produces a match and updates .lastIndex. The second invocation does not find a match and resets .lastIndex to zero.

If we create a regular expression specifically for .test(), then we probably won’t add /g. However, the likeliness of encountering /g increases if we use the same regular expression for replacing and for testing.

Once again, this problem also exists with /y:

> const regExp = /^X/y;
> regExp.test('Xa')
true
> regExp.test('Xa')
false
> regExp.test('Xa')
true
46.16.3.4 Pitfall 4: Code can produce unexpected results if .lastIndex isn’t zero

If an operation uses regular expression methods that are affected by .lastIndex, then we must ensure that .lastIndex is zero at the beginning. Otherwise, we may get unexpected results:

function countMatches(regExp, str) {
  let count = 0;
  while (regExp.test(str)) {
    count++;
  }
  return count;
}

const myRegExp = /a/g;
myRegExp.lastIndex = 4;
assert.equal(
  countMatches(myRegExp, 'babaa'), 1 // should be 3
);

Normally, .lastIndex is zero in newly created regular expressions and we won’t change it explicitly like we did in the example. But .lastIndex can still end up not being zero if we use the regular expression multiple times.

46.16.3.5 How to avoid the pitfalls of /g and /y

As an example of dealing with /g and .lastIndex, we revisit countMatches() from the previous example. How do we prevent a wrong regular expression from breaking our code? Let’s look at three approaches.

46.16.3.5.1 Throwing exceptions

First, we can throw an exception if /g isn’t set or .lastIndex isn’t zero:

function countMatches(regExp, str) {
  if (!regExp.global) {
    throw new Error('Flag /g of regExp must be set');
  }
  if (regExp.lastIndex !== 0) {
    throw new Error('regExp.lastIndex must be zero');
  }
  
  let count = 0;
  while (regExp.test(str)) {
    count++;
  }
  return count;
}
46.16.3.5.2 Cloning regular expressions

Second, we can clone the parameter. That has the added benefit that regExp won’t be changed.

function countMatches(regExp, str) {
  const cloneFlags = regExp.flags + (regExp.global ? '' : 'g');
  const clone = new RegExp(regExp, cloneFlags);

  let count = 0;
  while (clone.test(str)) {
    count++;
  }
  return count;
}
46.16.3.5.3 Using an operation that isn’t affected by .lastIndex or flags

Several regular expression operations are not affected by .lastIndex or by flags. For example, .match() ignores .lastIndex if /g is present:

function countMatches(regExp, str) {
  if (!regExp.global) {
    throw new Error('Flag /g of regExp must be set');
  }
  return (str.match(regExp) ?? []).length;
}

const myRegExp = /a/g;
myRegExp.lastIndex = 4;
assert.equal(countMatches(myRegExp, 'babaa'), 3); // OK!

Here, countMatches() works even though we didn’t check or fix .lastIndex.

46.16.4 Use case for .lastIndex: starting matching at a given index

Apart from storing the current position when matching multiple times, .lastIndex can also be used to:

We’ll look at examples next.

46.16.4.1 Example: Checking if a regular expression matches at a given index

Given that .test() is affected by /y and .lastIndex, we can use it to check if a regular expression regExp matches a string str at exactly a given index:

function startsWith(regExp, str, index) {
  if (!regExp.sticky || regExp.global) {
    throw new Error('Flag /y must be set. Flag /g must not be set.');
  }
  regExp.lastIndex = index;
  return regExp.test(str);
}
assert.equal(
  startsWith(/x+/y, 'aaxxx', 0), false
);
assert.equal(
  startsWith(/x+/y, 'aaxxx', 2), true
);

regExp is anchored to .lastIndex due to /y.

Note that we must not use the assertion ^ which would anchor regExp to the beginning of the input string.

46.16.4.2 Example: Finding the location of a match, starting at a given index

.search() lets us find the location where a regular expression matches:

> '#--#'.search(/#/)
0

Alas, we can’t change where .search() starts looking for matches. As a workaround, we can use .exec() for searching:

function searchFrom(regExp, str, index) {
  if (!regExp.global || regExp.sticky) {
    throw new Error('Flag /g must be set. Flag /y must not be set.');
  }
  regExp.lastIndex = index;
  const match = regExp.exec(str);
  if (match) {
    return match.index;
  } else {
    return -1;
  }
}

assert.equal(
  searchFrom(/#/g, '#--#', 0), 0
);
assert.equal(
  searchFrom(/#/g, '#--#', 1), 3
);
46.16.4.3 Example: Replacing an occurrence at a given index

When used without /g and with /y, .replace() makes one replacement – if there is a match at .lastIndex:

function replaceOnceAt(str, regExp, replacement, index) {
  if (!regExp.sticky || regExp.global) {
    throw new Error('Flag /y must be set. Flag /g must not be set.');
  }
  regExp.lastIndex = index;
  return str.replace(regExp, replacement);
}
assert.equal(
  replaceOnceAt('aa aaaa a', /a+/y, 'X', 0), 'X aaaa a')
;
assert.equal(
  replaceOnceAt('aa aaaa a', /a+/y, 'X', 3), 'aa X a'
);
assert.equal(
  replaceOnceAt('aa aaaa a', /a+/y, 'X', 8), 'aa aaaa X'
);

46.16.5 The downsides and an upside of .lastIndex

The regular expression property .lastIndex has two significant downsides:

On the upside, .lastIndex also gives us additional useful functionality: We can dictate where matching should begin (for some operations). Ideally, such functionality would be provided via an assertion such as \G (vs. a flag /y) and an index as an argument for various regular-expression-related methods (vs. a regular expression property .lastIndex). But it’s better than not having it at all.

46.17 RegExp.escape(): escaping text so that it can be used inside a regular expression ES2025

For a string text, RegExp.escape(text) creates a regular expression pattern that matches text.

Characters that have special meaning in regular expressions can’t be used verbatim and have to be escaped:

> RegExp.escape('(*)')
'\\(\\*\\)'

Note that we see each regular expression backslash twice: One of them is the actual backslash, the other one escapes it inside the string literal:

> '\\(\\*\\)' === String.raw`\(\*\)`
true

Characters that have no special meaning don’t have to be escaped:

> RegExp.escape('_abc123')
'_abc123'

46.17.1 Use case for RegExp.escape(): replacing all occurrences of a text

The classic use case for escaping was searching and replacing text:

function replacePlainText(str, searchText, replace) {
  const searchRegExp = new RegExp(
    RegExp.escape(searchText),
    'gu'
  );
  return str.replace(searchRegExp, replace)
}
assert.equal(
  replacePlainText('(a) and (a)', '(a)', '@'),
  '@ and @'
);

However, since ES2021, we have .replaceAll():

assert.equal(
  '(a) and (a)'.replaceAll('(a)', '@'),
  '@ and @'
);

46.17.2 Use case for RegExp.escape(): part of a regular expression must match a given text

The following code removes all occurrences of text inside str that are not quoted:

function removeUnquotedText(str, text) {
  const regExp = new RegExp(
    `(?<!“)${RegExp.escape(text)}(?!”)`,
    'gu'
  );
  return str.replaceAll(regExp, '•');
}
assert.equal(
  removeUnquotedText('“yes” and yes and “yes”', 'yes'),
  '“yes” and • and “yes”'
);

The same approach can also be used to find or count unquoted text.

Icon “exercise” Exercise: Searching for text that contains a wildcard

✅ exercises/regexps/includes-with-elision_test.mjs

46.18 Matching everything or nothing

Sometimes, we may need a regular expression that matches everything or nothing – for example, as a default value.

Regular expression literals can’t be empty because // starts a single-line comment. Therefore, the first of the previous two regular expressions is used in this case:

> new RegExp('')
/(?:)/

46.19 Tips for making regular expressions easier to use

In this section, we explore ways in which we can make regular expressions easier to use. We’ll use the following regular expression as an example:

const RE_API_SIGNATURE =
  /^(new |get )?([A-Za-z0-9_.\[\]]+)/;

Right now, it is still fairly cryptic. It will be much easier to understand once we get to “insignificant whitespace”.

46.19.1 Tip: Use flag /v

If we add flag /v to our regular expression, we get fewer quirks and more features:

const RE_API_SIGNATURE =
  /^(new |get )?([A-Za-z0-9_.\[\]]+)/v;

/v doesn’t change anything in this particular case, but it helps us if we add grapheme clusters with more than one code point or if we want features such as set operations in character classes.

46.19.2 Tip: order flags alphabetically

If there is more than one flag, we should order the flags alphabetically – e.g.:

/pattern/giv

That makes ordering consistent and is also how JavaScript displays regular expressions:

> String(/pattern/vgi)
'/pattern/giv'

46.19.3 Tip: Use named capture groups

Our regular expression contains two positional capture groups. If we name them, they describe their purposes and we need less external documentation:

const RE_API_SIGNATURE =
  /^(?<prefix>new |get )?(?<name>[A-Za-z0-9_.\[\]]+)/;

46.19.4 Tip: insignificant whitespace and line comments via #

So far, the regular expression is still fairly hard to read. We can change that by adding spaces and line breaks. Since regular expression literals don’t allow us to do that, we use the library Regex+ which provides us with the template tag regex:

import {regex} from 'regex';

const RE_API_SIGNATURE = regex`
  ^
  (?<prefix>
    new \x20  # constructor
    |
    get \x20  # getter
  )?
  (?<name>
    # Square brackets are needed for symbol keys
    [
      A-Z a-z 0-9 _
      .
      \[ \]
    ]+
  )
`;

The feature of ignoring whitespace in regular expression patterns is called insignificant whitespace. Additionally, we used a feature called inline comments – which are started by hash symbols (#).

Two observations:

In the future, JavaScript may get built-in support for insignificant whitespace via a flag /x (ECMAScript proposal).

With the regex template tag, the following flags are always active:

46.19.5 Tip: Write tests for your regular expression

To make sure that a regular expression works as intended, we can write tests for it. These are tests for RE_API_SIGNATURE:

assert.deepEqual(
  getCaptures(`get Map.prototype.size`),
  {
    prefix: 'get ',
    name: 'Map.prototype.size',
  }
);
assert.deepEqual(
  getCaptures(`new Array(len = 0)`),
  {
    prefix: 'new ',
    name: 'Array',
  }
);
assert.deepEqual(
  getCaptures(`Array.prototype.push(...items)`),
  {
    prefix: undefined,
    name: 'Array.prototype.push',
  }
);
assert.deepEqual(
  getCaptures(`Map.prototype[Symbol.iterator]()`),
  {
    prefix: undefined,
    name: 'Map.prototype[Symbol.iterator]',
  }
);

function getCaptures(apiSignature) {
  const match = RE_API_SIGNATURE.exec(apiSignature);
  // Spread so that the result does not have a null prototype
  // and is easier to compare.
  return {...match.groups};
}

46.19.6 Tip: Mention examples in your documentation

Seeing strings that match, helps with understanding what a regular expression is supposed to do:

/**
 * Matches API signatures – e.g.:
 * ```
 * `get Map.prototype.size`
 * `new Array(len = 0)`
 * `Array.prototype.push(...items)`
 * `Map.prototype[Symbol.iterator]()`
 * ```
 */
const RE_API_SIGNATURE = regex`
  ···
`;

Some documentation tools let us refer to unit tests in doc comments and show their code in the documentation. That’s a good alternative to what we have done above.

46.19.7 Bonus tip: Use interpolation to reuse patterns

The Regex+ library lets us interpolate regular expression fragments (“patterns”), which helps with reuse. The following example defines a simple markup syntax that is reminiscent of HTML:

import { pattern, regex } from 'regex';

const LABEL = pattern`[a-z\-]+`;
const ARGS = pattern`
  (?<args>
    \x20+
    ${LABEL}
  )*
`;
const NAME = pattern`
  (?<name> ${LABEL} )
`;

const TAG = regex`
  (?<openingTag>
    \[
    \x20*
    ${NAME}
    ${ARGS}
    \x20*
    \]
  )
  |
  (?<singletonTag>
    \[
    \x20*
    ${NAME}
    ${ARGS}
    \x20*
    / \]
  )
`;

assert.deepEqual(
  TAG.exec('[pre js line-numbers]').groups,
  {
    openingTag: '[pre js line-numbers]',
    name: 'pre',
    args: ' line-numbers',
    singletonTag: undefined,
    __proto__: null,
  }
);

assert.deepEqual(
  TAG.exec('[hr /]').groups,
  {
    openingTag: undefined,
    name: 'hr',
    args: undefined,
    singletonTag: '[hr /]',
    __proto__: null,
  }
);

The regular expression TAG uses the regular expression fragments NAME and ARGS twice – which reduces redundancy.

46.19.8 Bonus tip: insignificant whitespace without a library

With the following trick, we don’t need a library to write a regular expression with insignificant whitespace:

const RE_API_SIGNATURE = new RegExp(
  String.raw`
    ^
    (?<prefix>
      new \x20
      |
      get \x20
    )?
    (?<name>
      [
        A-Z a-z 0-9 _
        .
        \[ \]
      ]+
    )
  `.replaceAll(/\s+/g, ''), // (A)
  'v'
);
assert.equal(
  String(RE_API_SIGNATURE),
  String.raw`/^(?<prefix>new\x20|get\x20)?(?<name>[A-Za-z0-9_.\[\]]+)/v`
);

How does this code work?

We can even emulate inline comments like this:

// Template tag function
const cmt = () => '';
const RE = new RegExp(
  String.raw`
    a+ ${cmt`one or more as`}
  `.replaceAll(/\s+/g, ''),
  'v'
);
assert.equal(
  String(RE), '/a+/v'
);

Alas, it’s more syntactically noisy than I’d like.

46.19.9 Conclusion: This is how regular expressions are meant to be written

One reason why many people don’t like regular expressions is that they find them difficult to read. However, that is much less of a problem with insignificant whitespace and comments. I’d argue that is the proper way of writing regular expressions: Think what JavaScript code would look like if we had to write it without whitespace and comments.

46.20 Quick reference: regular expression functionality

46.20.1 Summary: .global (/g) and .sticky (/y)

The following two methods are completely unaffected by /g and /y:

This table explains how the remaining regular-expression-related methods are affected by these two flags (if neither /g nor /y are there, regExp.lastIndex is always ignored):

FlagsHonors .lastIndex?Updates .lastIndex?
s.match/y✔✔
/g /gy✘0
s.matchAll/g /gy✔✘
r.exec/g /y /gy✔✔
s.replace/y✔✔
/g /gy✘0
s.replaceAll/g /gy✘0
r.test/g /y /gy✔✔

Icon “external” A longer overview

I posted a longer table online that was created via a Node.js script.

46.20.2 String.prototype.*: matching and searching

46.20.3 String.prototype.*: splitting and replacing

46.20.4 RegExp.prototype.*

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