Regular expressions

Regular expressions are a important concept in formal language theory. They are a way to describe a possibly infinite set of character strings (called a language). A regular expression, at its core, needs the following features:

Assuming a finite alphabet (such as the 26 letters of the English alphabet, or the entire Unicode character set), all regular languages can be generated by the features above. Of course, many patterns are very tedious to express this way (such as "10 digits" or "a character that's not a space"), so JavaScript regular expressions include many shorthands, introduced below.

A regular expression is typically created as a literal by enclosing a pattern in forward slashes (/):

const regex1 = /ab+c/g;

Regular expressions can also be created with the RegExp() constructor:

const regex2 = new RegExp("ab+c", "g");

They have no runtime differences, although they may have implications on performance, static analyzability, and authoring ergonomic issues with escaping characters. For more information, see the RegExp reference.

Flags are special parameters that can change the way a regular expression is interpreted or the way it interacts with the input text. Each flag corresponds to one accessor property on the RegExp object.

The i, m, and s flags can be enabled or disabled for specific parts of a regex using the modifier syntax.

The sections below list all available regex syntaxes, grouped by their syntactic nature.

Assertions are constructs that test whether the string meets a certain condition at the specified position, but not consume characters. Assertions cannot be quantified.

Atoms are the most basic units of a regular expression. Each atom consumes one or more characters in the string, and either fails the match or allows the pattern to continue matching with the next atom.

These features do not specify any pattern themselves, but are used to compose patterns.

Escape sequences in regexes refer to any kind of syntax formed by \ followed by one or more characters. They may serve very different purposes depending on what follow \. Below is a list of all valid "escape sequences":

\ followed by 0 and another digit becomes a legacy octal escape sequence, which is forbidden in Unicode-aware mode. \ followed by any other digit sequence becomes a backreference.

In addition, \ can be followed by some non-letter-or-digit characters, in which case the escape sequence is always a character escape representing the escaped character itself:

The other ASCII characters, namely space character, ", ', _, and any letter character not mentioned above, are not valid escape sequences. In Unicode-unaware mode, escape sequences that are not one of the above become identity escapes: they represent the character that follows the backslash. For example, \a represents the character a. This behavior limits the ability to introduce new escape sequences without causing backward compatibility issues, and is therefore forbidden in Unicode-aware mode.