The value of the pattern attribute in an input element is a regular expression. This regular expression is used to define what constitutes a valid value for the input.

Regular Expressions: Regular expressions (regex) are sequences of characters that define search patterns. They are commonly used for string matching and validation.

Usage Example:

<input type="text" pattern="\d{5}" placeholder="Enter a 5-digit number">

Here, the pattern attribute value is a regular expression that validates a five-digit number.

References:

MDN Web Docs on pattern

Regular Expressions Documentation