A regular expression, also known as regex or regexp, is a sequence of characters that forms a pattern. It is often used in programming languages, including JavaScript, to perform pattern matching and searching tasks.

In JavaScript, regular expressions are defined using a combination of characters and metacharacters that represent specific patterns of text. For example, the regular expression /hello/ would match any instance of the word "hello" in a text string.

• ^: Matches the beginning of a line or string.

• $: Matches the end of a line or string.

• .: Matches any single character except for new line.

• *: Matches zero or more occurrences of the preceding character or group.

• +: Matches one or more occurrences of the preceding character or group.

• ?: Matches zero or one occurrences of the preceding character or group.

• (): Defines a capturing group that can be used for backreferences.

Regular expressions can also be used to validate and format input from users. For example, a regular expression can be used to ensure that a user has entered a valid email address or phone number.

Overall, understanding regular expressions is an important skill for any JavaScript programmer, as it allows for powerful text manipulation and validation capabilities.