Sets and ranges [...]

Several characters or character classes inside square brackets […] mean to “search for any character among given”.

Sets

For instance, [eao] means any of the 3 characters: 'a', 'e', or 'o'.

That’s called a set. Sets can be used in a regexp along with regular characters:

// find [t or m], and then "op"
alert( "Mop top".match(/[tm]op/gi) ); // "Mop", "top"

Please note that although there are multiple characters in the set, they correspond to exactly one character in the match.

So the example below gives no matches:

// find "V", then [o or i], then "la"
alert( "Voila".match(/V[oi]la/) ); // null, no matches

The pattern searches for:

V,
then one of the letters [oi],
then la.

So there would be a match for Vola or Vila.

Ranges

Square brackets may also contain character ranges.

For instance, [a-z] is a character in range from a to z, and [0-5] is a digit from 0 to 5.

In the example below we’re searching for "x" followed by two digits or letters from A to F:

alert( "Exception 0xAF".match(/x[0-9A-F][0-9A-F]/g) ); // xAF

Here [0-9A-F] has two ranges: it searches for a character that is either a digit from 0 to 9 or a letter from A to F.

If we’d like to look for lowercase letters as well, we can add the range a-f: [0-9A-Fa-f]. Or add the flag i.

We can also use character classes inside […].

For instance, if we’d like to look for a wordly character \w or a hyphen -, then the set is [\w-].

Combining multiple classes is also possible, e.g. [\s\d] means “a space character or a digit”.

For instance:

\d – is the same as [0-9],
\w – is the same as [a-zA-Z0-9_],
\s – is the same as [\t\n\v\f\r ], plus few other rare unicode space characters.

Example: multi-language \w

As the character class \w is a shorthand for [a-zA-Z0-9_], it can’t find Chinese hieroglyphs, Cyrillic letters, etc.

We can write a more universal pattern, that looks for wordly characters in any language. That’s easy with unicode properties: [\p{Alpha}\p{M}\p{Nd}\p{Pc}\p{Join_C}].

Let’s decipher it. Similar to \w, we’re making a set of our own that includes characters with following unicode properties:

Alphabetic (Alpha) – for letters,
Mark (M) – for accents,
Decimal_Number (Nd) – for digits,
Connector_Punctuation (Pc) – for the underscore '_' and similar characters,
Join_Control (Join_C) – two special codes 200c and 200d, used in ligatures, e.g. in Arabic.

An example of use:

let regexp = /[\p{Alpha}\p{M}\p{Nd}\p{Pc}\p{Join_C}]/gu;

let str = `Hi 你好 12`;

// finds all letters and digits:
alert( str.match(regexp) ); // H,i,你,好,1,2

Of course, we can edit this pattern: add unicode properties or remove them. Unicode properties are covered in more details in the article 유니코드: 'u' 플래그와 \p{...} 클래스.

Unicode properties p{…} are not yet implemented in Edge and Firefox. If we really need them, we can use library XRegExp.

Or just use ranges of characters in a language that interests us, e.g. [а-я] for Cyrillic letters.

Excluding ranges

Besides normal ranges, there are “excluding” ranges that look like [^…].

They are denoted by a caret character ^ at the start and match any character except the given ones.

For instance:

[^aeyo] – any character except 'a', 'e', 'y' or 'o'.
[^0-9] – any character except a digit, the same as \D.
[^\s] – any non-space character, same as \S.

The example below looks for any characters except letters, digits and spaces:

alert( "alice15@gmail.com".match(/[^\d\sA-Z]/gi) ); // @ and .

Escaping in […]

Usually when we want to find exactly a special character, we need to escape it like \.. And if we need a backslash, then we use \\, and so on.

In square brackets we can use the vast majority of special characters without escaping:

Symbols . + ( ) never need escaping.
A hyphen - is not escaped in the beginning or the end (where it does not define a range).
A caret ^ is only escaped in the beginning (where it means exclusion).
The closing square bracket ] is always escaped (if we need to look for that symbol).

In other words, all special characters are allowed without escaping, except when they mean something for square brackets.

A dot . inside square brackets means just a dot. The pattern [.,] would look for one of characters: either a dot or a comma.

In the example below the regexp [-().^+] looks for one of the characters -().^+:

// No need to escape
let regexp = /[-().^+]/g;

alert( "1 + 2 - 3".match(regexp) ); // Matches +, -

…But if you decide to escape them “just in case”, then there would be no harm:

// Escaped everything
let regexp = /[\-\(\)\.\^\+]/g;

alert( "1 + 2 - 3".match(regexp) ); // also works: +, -

Ranges and flag “u”

If there are surrogate pairs in the set, flag u is required for them to work correctly.

For instance, let’s look for [𝒳𝒴] in the string 𝒳:

alert( '𝒳'.match(/[𝒳𝒴]/) ); // shows a strange character, like [?]
// (the search was performed incorrectly, half-character returned)

The result is incorrect, because by default regular expressions “don’t know” about surrogate pairs.

The regular expression engine thinks that [𝒳𝒴] – are not two, but four characters:

left half of 𝒳 (1),
right half of 𝒳 (2),
left half of 𝒴 (3),
right half of 𝒴 (4).

We can see their codes like this:

for(let i=0; i<'𝒳𝒴'.length; i++) {
  alert('𝒳𝒴'.charCodeAt(i)); // 55349, 56499, 55349, 56500
};

So, the example above finds and shows the left half of 𝒳.

If we add flag u, then the behavior will be correct:

alert( '𝒳'.match(/[𝒳𝒴]/u) ); // 𝒳

The similar situation occurs when looking for a range, such as [𝒳-𝒴].

If we forget to add flag u, there will be an error:

'𝒳'.match(/[𝒳-𝒴]/); // Error: Invalid regular expression

The reason is that without flag u surrogate pairs are perceived as two characters, so [𝒳-𝒴] is interpreted as [<55349><56499>-<55349><56500>] (every surrogate pair is replaced with its codes). Now it’s easy to see that the range 56499-55349 is invalid: its starting code 56499 is greater than the end 55349. That’s the formal reason for the error.

With the flag u the pattern works correctly:

// look for characters from 𝒳 to 𝒵
alert( '𝒴'.match(/[𝒳-𝒵]/u) ); // 𝒴