Backreference in RegEx allows matching the same text as a previous capturing group

A capturing group (...) groups a subpattern. For example, ([A-Z]\d) matches a letter followed by a digit, like "A1". A backreference, denoted by \1, \2, etc., refers to a previous capturing group and matches the same text as that group.

For instance, in the pattern ([A-Z]\d)\1, \1 refers to the first capturing group ([A-Z]\d), ensuring that the same letter-digit combination is repeated.

const pattern = /([A-Z]\d)\1/;

pattern.test("A1A1"); // true
pattern.test("A1B1"); // false

You can also use a backreference with quantifiers:

const pattern = /(\d)\1{2}/;

pattern.test("111"); // true
pattern.test("122"); // false