N

A positive integer referring to the number of a capturing group.

A backreference is a way to match the same text as previously matched by a capturing group. Capturing groups count from 1, so the first capturing group's result can be referenced with\1, the second with\2, and so on.\0 is acharacter escape for the NUL character.

Incase-insensitive matching, the backreference may match text with different casing from the original text.

/(b)\1/i.test("bB"); // true

The backreference must refer to an existent capturing group. If the number it specifies is greater than the total number of capturing groups, a syntax error is thrown.

/(a)\2/u; // SyntaxError: Invalid regular expression: Invalid escape

InUnicode-unaware mode, invalid backreferences become alegacy octal escape sequence. This is adeprecated syntax for web compatibility, and you should not rely on it.

/(a)\2/.test("a\x02"); // true

If the referenced capturing group is unmatched (for example, because it belongs to an unmatched alternative in adisjunction), or the group hasn't matched yet (for example, because it lies to the right of the backreference), the backreference always succeeds (as if it matches the empty string).

/(?:a|(b))\1c/.test("ac"); // true/\1(a)/.test("a"); // true

The following function matches thetitle='xxx' andtitle="xxx" patterns in a string. To ensure the quotes match, we use a backreference to refer to the first quote. Accessing the second capturing group ([2]) returns the string between the matching quote characters:

function parseTitle(metastring) {  return metastring.match(/title=(["'])(.*?)\1/)[2];}parseTitle('title="foo"'); // 'foo'parseTitle("title='foo' lang='en'"); // 'foo'parseTitle('title="Named capturing groups\' advantages"'); // "Named capturing groups' advantages"

The following function finds duplicate words in a string (which are usually typos). Note that it uses the\wcharacter class escape, which only matches English letters but not any accented letters or other alphabets. If you want more generic matching, you may want tosplit the string by whitespace and iterate over the resulting array.

function findDuplicates(text) {  return text.match(/\b(\w+)\s+\1\b/i)?.[1];}findDuplicates("foo foo bar"); // 'foo'findDuplicates("foo bar foo"); // undefinedfindDuplicates("Hello hello"); // 'Hello'findDuplicates("Hello hellos"); // undefined

• Groups and backreferences guide
• Regular expressions
• Capturing group:(...)
• Named capturing group:(?<name>...)
• Named backreference:\k<name>