java: search & replace in a Stream

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.

I got some good ideas from the link provided and ended up writing a small class to handle replacement of $VAR$ variables in a stream. For posterity:

public class ReplacingOutputStream extends OutputStream {
    private static final int DOLLAR_SIGN = "$".codePointAt(0);
    private static final int BACKSLASH = "\\".codePointAt(0);
    private final OutputStream delegate;
    private final Map<String, Object> replacementValues;

    private int previous = Integer.MIN_VALUE;
    private boolean replacing = false;
    private ArrayList<Integer> replacement = new ArrayList<Integer>();


    public ReplacingOutputStream(OutputStream delegate, Map<String, Object> replacementValues) {
        this.delegate = delegate;
        this.replacementValues = replacementValues;
    }

    public @Override void write(int b) throws IOException {
        if (b == DOLLAR_SIGN && previous != BACKSLASH) {
            if (replacing) {
                doReplacement();
                replacing = false;
            } else {
                replacing = true;
            }
        } else {
            if (replacing) {
                replacement.add(b);
            } else {
                delegate.write(b);
            }
        }

        previous = b;
    }

    private void doReplacement() throws IOException {
        StringBuilder sb = new StringBuilder();
        for (Integer intval : replacement) {
            sb.append(Character.toChars(intval));
        }
        replacement.clear();

        String oldValue = sb.toString();
        Object _newValue = replacementValues.get(oldValue);
        if (_newValue == null) {
            throw new RuntimeException("Could not find replacement variable for value '"+oldValue+"'.");
        }

        String newValue = _newValue.toString();
        for (int i=0; i < newValue.length(); ++i) {
            int value = newValue.codePointAt(i);
            delegate.write(value);
        }
    }
}