Regular expressions are a powerful tool used in computing for searching, matching, and manipulating text. They are used to find patterns in text data, and are commonly used in programming languages like JavaScript, Python, and Java.

Regular expressions can be used for values of type string, but they can also be used for other types of data as well, including numbers. This is because regular expressions are based on the concept of patterns, which can be used to match specific sequences of characters or digits.

There are several reasons why regular expressions are useful for working with text data:

1. Pattern matching: Regular expressions allow you to search for specific patterns within a string of text. For example, you could use a regular expression to search for all instances of a particular word or phrase within a document.

2. Text manipulation: Regular expressions can also be used to replace or modify text data. For example, you could use a regular expression to replace all instances of a certain word with a different word.

3. Data validation: Regular expressions can be used to validate input data, such as email addresses, phone numbers, or credit card numbers. This can help ensure that the data is in the correct format before it is used in a program or application.

In conclusion, regular expressions are a powerful tool for working with text data, and can be used for values of type string as well as other types of data, including numbers. They are widely used in programming languages and are an essential skill for any programmer working with text data.