SUBSTITUTE THE LAST OCCURANCE OF A TEXT IN A STRING

Text               :  this is a isotype

Search Text  :  "is"

Replace with :  "@@"

Formula        : 
=SUBSTITUTE(A1,"is","@@",INT(LEN(A1)-LEN(SUBSTITUTE(A1,"is","")))/2)

Result             : this is a @@type

Syntax

SUBSTITUTE(text, old_text, new_text, [instance_num])

The SUBSTITUTE function syntax has the following arguments (argument: A value that provides information to an action, an event, a method, a property, a function, or a procedure.):

• Text  Required. The text or the reference to a cell containing text for which you want to substitute characters.
• Old_text  Required. The text you want to replace.
• New_text  Required. The text you want to replace old_text with.
• Instance_num  Optional. Specifies which occurrence of old_text you want to replace with new_text. If you specify instance_num, only that instance of old_text is replaced. Otherwise, every occurrence of old_text in text is changed to new_text.