regexp replace in a stream, buffering only the necessary amount of bytes, but all if there is no match

rxstream.go

/*
Try:

	cat | go run rxstream.go '(?ms)(.*)42' '{"chars before 42": "$1"}'

And input:

	abc42abc42

*/
package main

import (
	"io"
	"os"
	"regexp"
)

// replace with pattern
// returns the number of matches
func streamReplaceRx(
	w io.Writer,
	r io.Reader,
	rx *regexp.Regexp,
	replacement []byte,
	readBuf []byte,
) (int, error) {
	var (
		matchCount int
		matchBuf   []byte
		eof        bool
	)

readLoop:
	for {
		if eof {
			// output the remaining buffer
			_, err := w.Write(matchBuf)
			return matchCount, err
		}

		n, err := r.Read(readBuf)
		if err != nil {
			if err == io.EOF {
				// depending on the reader implementation, we cannot be sure if any bytes were
				// read, so we need to do a matching round before returning
				eof = true
				err = nil
			} else {
				return matchCount, err
			}
		}

		// store what was read
		matchBuf = append(matchBuf, readBuf[:n]...)

		// find all the matches in the currently available input
		for {
			// find the next match, with submatches for template replacement
			m := rx.FindSubmatchIndex(matchBuf)

			// if there is no match, we need to read more, because we don't know if the expression
			// could match a longer input
			if len(m) == 0 {
				continue readLoop
			}

			matchCount++

			// the unmatched part of the input
			if _, err := w.Write(matchBuf[:m[0]]); err != nil {
				return matchCount, err
			}

			// avoid looping around zero-length matches by reading more
			if m[0] == m[1] {
				continue readLoop
			}

			// the replacement
			if _, err := w.Write(rx.Expand(nil, replacement, matchBuf, m)); err != nil {
				return matchCount, err
			}

			// remove until the matched part and continue checking for matches in the available
			// input
			matchBuf = matchBuf[m[1]:]
		}
	}
}

func main() {
	if len(os.Args) < 3 {
		os.Exit(2)
	}

	rx := regexp.MustCompile(os.Args[1])

	readBuf := make([]byte, 4)
	streamReplaceRx(
		os.Stdout,
		os.Stdin,
		rx,
		[]byte(os.Args[2]),
		readBuf,
	)
}