Search and Replace Strings in a Text Box

I need to upon the click of a button(I can induce via click event) take several sentences (lets say 5) that are stored within 5 variables (CLASS LEVEL) and replace certain sentences within a text box with those. for instance......

Text box has in it:

asdfasdfasdfasdfsentence1sdfsdfsd
fsdfsdfsentence2asdfsdfsd
fsdfsdsentence3sdfsdfsdf

etc

I need to take the values in var1 var 2 and var 3 and repalce sentence 1 2 and 3 with those values as strings.... (note: the vars hold values that were placed in text boxes etc)

I have no idea where to start......... Basically what I am trying to do is I have the source code for a webpage in HTML stored in a multiline text box and I need to substituen image urls etc for placholder strings i have in the code, so when this code runs it replaces all the placeholders and leaves me with clean html code... anyone have any ideas, i am a totally newbie