How do I do on-the-fly search & replace in a Java Stream (input or output)?
I don't want to load the stream into memory or to a file.
I just see the bytes passing by and I need to do some replacements. The sequences being replaced are short (up to 20 bytes).
You could implement a deterministic finite automaton which looks at each byte once only (e.g. no lookbehind is required), so that you would basically stream the input through a buffer holding max as many characters as the length of your pattern, outputting the pattern on a match or overflowing (non-matched) characters when advancing in the pattern. Runtime is linear after preparation of the pattern.
Wikipedia has some information on pattern matching and how that works in theory.
You can use the class provided here if static replacement rules are enough for you.
This is not really an answer to your question.
You should though, look into the java.nio package.
Take a look at the following examples:
The first example shows how to do a simple "grep" on a file.
Using the NIO you will not have to worry about a buffer size, just let the regular expression library methods do the heavy lifting.