Character class: [...], [^...]

operand1, operand2

Can be a single character, another square-bracket-enclosed character class, a character class escape, a Unicode character class escape, or a string using the \q syntax.

substring

A literal string.

A character class specifies a list of characters between square brackets and matches any character in the list. The v flag drastically changes how character classes are parsed and interpreted. The following syntaxes are available in both v mode and non-v mode:

• A single character: matches the character itself.
• A range of characters: matches any character in the inclusive range. The range is specified by two characters separated by a dash (-). The first character must be smaller in character value than the second character. The character value is the Unicode code point of the character. Because Unicode code points are usually assigned to alphabets in order, [a-z] specifies all lowercase Latin characters, while [α-ω] specifies all lowercase Greek characters. In Unicode-unaware mode, regexes are interpreted as a sequence of BMP characters. Therefore, surrogate pairs in character classes represent two characters instead of one; see below for details.
• Escape sequences: \b, \-, character class escapes, Unicode character class escapes, and other character escapes.

These syntaxes can occur any number of times, and the character sets they represent are unioned. For example, /[a-zA-Z0-9]/ matches any letter or digit.

The ^ prefix in a character class creates a complement class. For example, [^abc] matches any character except a, b, or c. The ^ character is a literal character when it appears in the middle of a character class — for example, [a^b] matches the characters a, ^, and b.

The lexical grammar does a very rough parse of regex literals, so that it does not end the regex literal at a / character which appears within a character class. This means /[/]/ is valid without needing to escape the /.

The boundaries of a character range must not specify more than one character, which happens if you use a character class escape. For example:

/[\s-9]/u; // SyntaxError: Invalid regular expression: Invalid character class

In Unicode-unaware mode, character ranges where one boundary is a character class makes the - become a literal character. This is a deprecated syntax for web compatibility, and you should not rely on it.

/[\s-9]/.test("-"); // true

In Unicode-unaware mode, regexes are interpreted as a sequence of BMP characters. Therefore, surrogate pairs in character classes represent two characters instead of one.

/[😄]/.test("\ud83d"); // true
/[😄]/u.test("\ud83d"); // false

/[😄-😛]/.test("😑"); // SyntaxError: Invalid regular expression: /[😄-😛]/: Range out of order in character class
/[😄-😛]/u.test("😑"); // true

Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. For example, the pattern /[E-F]/i only matches E, F, e, and f, while the pattern /[E-f]/i matches all uppercase and lowercase ASCII letters (because it spans over E–Z and a–f), as well as [, \, ], ^, _, and `.

Non-v-mode character classes interpret most character literally and have less restrictions about the characters they can contain. For example, . is the literal dot character, not the wildcard. The only characters that cannot appear literally are \, ], and -.

• In character classes, most escape sequences are supported, except \b, \B, and backreferences. \b indicates a backspace character instead of a word boundary, while the other two cause syntax errors. To use \ literally, escape it as \\.
• The ] character indicates the end of the character class. To use it literally, escape it as \].
• The dash (-) character, when used between two characters, indicates a range. When it appears at the start or end of a character class, it is a literal character. It's also a literal character when it's used in the boundary of a range. For example, [a-] matches the characters a and -, [!--] matches the characters ! to -, and [--9] matches the characters - to 9. You can also escape it as \- if you want to use it literally anywhere.

The basic idea of character classes in v mode remains the same: you can still use most characters literally, use - to denote character ranges, and use escape sequences. One of the most important features of the v flag is set notation within character classes. As previously mentioned, normal character classes can express unions by concatenating two ranges, such as using [A-Z0-9] to mean "the union of the set [A-Z] and the set [0-9]". However, there's no easy way to represent other operations with character sets, such as intersection and difference.

With the v flag, intersection is expressed with &&, and subtraction with --. The absence of both implies union. The two operands of && or -- can be a character, character escape, character class escape, or even another character class. For example, to express "a word character that's not an underscore", you can use [\w--_]. You cannot mix operators on the same level. For example, [\w&&[A-z]--_] is a syntax error. However, because you can nest character classes, you can be explicit by writing [\w&&[[A-z]--_]] or [[\w&&[A-z]]--_] (which both mean [A-Za-z]). Similarly, [AB--C] is invalid and you need to write [A[B--C]] (which just means [AB]).

In v mode, the Unicode character class escape \p can match finite-length strings, such as emojis. For symmetry, regular character classes can also match more than one character. To write a "string literal" in a character class, you wrap the string in \q{...}. The only regex syntax supported here is disjunction — apart from this, \q must completely enclose literals (including escaped characters). This ensures that character classes can only match finite-length strings with finitely many possibilities.

Because the character class syntax is now more sophisticated, more characters are reserved and forbidden from appearing literally.

• In addition to ] and \, the following characters must be escaped in character classes if they represent literal characters: (, ), [, {, }, /, -, |. This list is somewhat similar to the list of syntax characters, except that ^, $, *, +, and ? are not reserved inside character classes, while / and - are not reserved outside character classes (although / may delimit a regex literal and therefore still needs to be escaped). All these characters may also be optionally escaped in u-mode character classes.
• The following "double punctuator" sequences must be escaped as well (but they don't make much sense without the v flag anyway): &&, !!, ##, $$, %%, **, ++, ,,, .., ::, ;;, <<, ==, >>, ??, @@, ^^, ``, ~~. In u mode, some of these characters can only appear literally within character classes and cause a syntax error when escaped. In v mode, they must be escaped when appearing in pairs, but can be optionally escaped when appearing alone. For example, /[\!]/u is invalid because it's an identity escape, but both /[\!]/v and /[!]/v are valid, while /[!!]/v is invalid. The literal character reference has a detailed table of which characters can appear escaped or unescaped.

Complement character classes [^...] cannot possibly be able to match strings longer than one character. For example, [\q{ab|c}] is valid and matches the string "ab", but [^\q{ab|c}] is invalid because it's unclear how many characters should be consumed. The check is done by checking if all \q contain single characters and all \p specify character properties — for unions, all operands must be purely characters; for intersections, at least one operand must be purely characters; for subtraction, the leftmost operand must be purely characters. The check is syntactic without looking at the actual character set being specified, which means although /[^\q{ab|c}--\q{ab}]/v is equivalent to /[^c]/v, it's still rejected.

Case-insensitive matching works by case-folding both the expected character set and the matched string. When specifying complement classes, the order in which JavaScript performs case-folding and complementing is important. In brief, [^...] in u mode matches allCharacters - caseFold(original), while in v mode matches caseFold(allCharacters) - caseFold(original). This ensures that all complement class syntaxes, including [^...], \P, \W, etc., cancel each other out.

Consider the following two regexes (to simplify things, let's assume that Unicode characters are one of three kinds: lowercase, uppercase, and caseless, and each uppercase letter has a unique lowercase counterpart, and vice versa):

const r1 = /\p{Lowercase_Letter}/iu;
const r2 = /[^\P{Lowercase_Letter}]/iu;

The r2 is a double negation and seems to be equivalent with r1. But in fact, r1 matches all lower- and uppercase ASCII letters, while r2 matches none. Here's a step-by-step explanation:

• In r1, \p{Lowercase_Letter} constructs a set of all lowercase characters. Characters in this set are then case-folded to their lowercase form, so they stay the same. The input string is also case-folded to lowercase. Therefore, "A" and "a" are both folded to "a" and matched by r1.
• In r2, \P{Lowercase_Letter} first constructs a set of all non-lowercase characters, i.e., uppercase letters and caseless characters. Characters in this set are then case-folded to their lowercase form, so the character set becomes all lowercase letters and caseless characters. [^...] negates the match, causing it to match anything that's not in this set, i.e., an uppercase letter. However, the input is still case-folded to lowercase, so "A" is folded to "a" and not matched by r2.

The main observation here is that after [^...] negates the match, the expected character set may not be a subset of the set of case-folded Unicode characters, causing the case-folded input to not be in the expected character set. In v mode, the set of all characters is also case-folded. The \P character class itself also works slightly differently in v mode (see Unicode character class escape). All of these ensure that [^\P{Lowercase_Letter}] and \p{Lowercase_Letter} are strictly equivalent.

The following function determines whether a string contains a valid hexadecimal number:

function isHexadecimal(str) {
  return /^[0-9A-F]+$/i.test(str);
}

isHexadecimal("2F3"); // true
isHexadecimal("beef"); // true
isHexadecimal("undefined"); // false

The following function matches Greek letters.

function greekLetters(str) {
  return str.match(/[\p{Script_Extensions=Greek}&&\p{Letter}]/gv);
}

// 𐆊 is U+1018A GREEK ZERO SIGN
greekLetters("π𐆊P0零αAΣ"); // [ 'π', 'α', 'Σ' ]

The following function matches all non-ASCII numbers.

function nonASCIINumbers(str) {
  return str.match(/[\p{Decimal_Number}--[0-9]]/gv);
}

// 𑜹 is U+11739 AHOM DIGIT NINE
nonASCIINumbers("𐆊0零1𝟜𑜹a"); // [ '𝟜', '𑜹' ]

The following function matches all line terminator sequences, including the line terminator characters and the sequence \r\n (CRLF).

function getLineTerminators(str) {
  return str.match(/[\r\n\u2028\u2029\q{\r\n}]/gv);
}

getLineTerminators(`
A poem\r
Is split\r\n
Into many
Stanzas
`); // [ '\r', '\r\n', '\n' ]

This example is exactly equivalent to /(?:\r|\n|\u2028|\u2029|\r\n)/gu or /(?:[\r\n\u2028\u2029]|\r\n)/gu, except shorter.

The most useful case of \q{} is when doing subtraction and intersection. Previously, this was possible with multiple lookaheads. The following function matches flags that are not one of the American, Chinese, Russian, British, and French flags.

function notUNSCPermanentMember(flag) {
  return /^[\p{RGI_Emoji_Flag_Sequence}--\q{🇺🇸|🇨🇳|🇷🇺|🇬🇧|🇫🇷}]$/v.test(flag);
}

notUNSCPermanentMember("🇺🇸"); // false
notUNSCPermanentMember("🇩🇪"); // true

This example is mostly equivalent to /^(?!🇺🇸|🇨🇳|🇷🇺|🇬🇧|🇫🇷)\p{RGI_Emoji_Flag_Sequence}$/v, except perhaps more performant.

• Character classes guide
• Regular expressions
• Character class escape: \d, \D, \w, \W, \s, \S
• Unicode character class escape: \p{...}, \P{...}
• Literal character: a, b
• Character escape: \n, \u{...}
• Disjunction: |
• RegExp v flag with set notation and properties of strings on v8.dev (2022)