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

A character class specifies a list of characters between square brackets and matches any character in the list. The following syntaxes are available:

• 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 non-unicode 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.

Unlike other parts of the regex, 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 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 non-unicode 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 non-unicode 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 `.

The ^ prefix in a character class inverts the match. 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 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

• Character classes
• Regular expressions reference
• Character class escape: \d, \D, \w, \W, \s, \S
• Unicode character class escape: \p{...}, \P{...}
• Literal character: a, b
• Character escape: \n, \u{...}
• Disjunction: |