The JSTL <c:replace> tag is used to replace all occurrences of a specified string in a given string with another specified string. This tag is part of the JSTL core tag library, and is typically used in JSP pages.

The basic syntax for the <c:replace> tag is as follows:

<c:replace string="stringToReplace"
           substring="substringToReplace"
           replacement="replacementString" />

The attributes of the <c:replace> tag are as follows:

• string: The string in which to replace the substring. This attribute is required.
• substring: The substring to be replaced. This attribute is required.
• replacement: The string to replace the substring with. This attribute is required.

Here's an example that shows how to use the <c:replace> tag to replace all occurrences of the substring "world" in the string "Hello, world!" with the string "Java":

<c:set var="myString" value="Hello, world!" />
<c:replace string="${myString}" substring="world" replacement="Java" />

In this example, the <c:set> tag sets the value of the "myString" variable to "Hello, world!". The <c:replace> tag then replaces all occurrences of the substring "world" in the "myString" variable with the string "Java".

The resulting output of this code will be:

Hello, Java!

Note that the <c:replace> tag does not modify the original string, but rather returns a new string with the replacements made. If you want to modify the original string, you can use the JSTL <c:set> tag to set the value of the original string to the result of the <c:replace> tag:

<c:set var="myString" value="Hello, world!" />
<c:set var="myString" value="<c:replace string="${myString}" substring="world" replacement="Java" />" />

In this example, the <c:set> tag sets the value of the "myString" variable to the result of the <c:replace> tag, effectively modifying the original string.