In 1.3 you don't get String.replace() but you can write your own using indexOf() and substring() to find the delimiters and break up the bits. Here's part of one I did in my first J++ project:
Ouch, this is pretty bad!

See if you can make it a lot better by keeping the output separate from the input. This also loops until the heap explodes if "new" contains "old", eg replace "BERT" by "ROBERT".