private static final Charset UTF8 = Charset.forName("UTF-8");
private static final  Pattern HEADER_PATTERN =
     Pattern.compile("<tag>\s<meta [^>]*>\s</tag>", Pattern.MULTILINE);

FileInputStream input = new FileInputStream(inputFile);
FileChannel channel = input.getChannel();
ByteBuffer bbuf = channel.map(FileChannel.MapMode.READ_ONLY,
    0, (int) channel.size());
CharBuffer cbuf = UTF8.newDecoder().decode(bbuf);
Matcher matcher = HEADER_PATTERN.matcher(cbuf);
StringBuffer sb = new StringBuffer();
while (matcher.find()) {
    matcher.appendReplacement(sb, "Some replacement text");
}
matcher.appendTail(sb);
IOUtils.closeQuietly(input);
IOUtils.write(sb, new FileOutputStream(inputFile), UTF8);