The JSTL (JavaServer Pages Standard Tag Library) fmt tag library provides a set of tags for formatting and internationalizing data in JSP (JavaServer Pages) pages. One of the most commonly used tags in the fmt tag library is the <fmt:parseNumber> tag, which is used for parsing numbers and converting them to a specific format.

The <fmt:parseNumber> tag takes a string representation of a number and parses it into a java.lang.Number object, which can then be formatted using other tags in the fmt tag library. The syntax of the tag is as follows:

<fmt:parseNumber value="numberString" [type="numberType"] [var="varName"] />

where numberString is the string representation of the number to be parsed, numberType is the type of the number to be parsed (if not specified, the default type for the current locale is used), and varName is the name of the variable to store the parsed number.

For example, to parse the number "123.45" using the default type for the current locale, you would use the following tag:

<fmt:parseNumber value="123.45" var="parsedNumber" />

This tag parses the number string "123.45" into a java.lang.Number object and stores it in the variable parsedNumber.

You can also specify a custom number type to parse the number. The supported number types are byte, short, int, long, float, and double. For example, to parse the number "123.45" as a float, you would use the following tag:

<fmt:parseNumber value="123.45" type="float" var="parsedNumber" />

This tag parses the number string "123.45" as a float and stores the parsed number in the variable parsedNumber.

Note that the fmt tag library must be imported at the beginning of the JSP page using the following tag:

<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>

This imports the fmt tag library and assigns it a prefix of fmt.