appendReplacement doesn't throw for invalid replacement groups

[cushon]

appendReplacement doesn't throw an exception for invalid replacement groups, which is inconsistent with java.util.regex. Is this deliberate, or should the behaviour match java.util.regex?

    String pattern = "(.*)";
    String input = "hello";
    {
      System.err.println("> re2j");
      var p = com.google.re2j.Pattern.compile(pattern);
      var m = p.matcher(input);
      StringBuilder sb = new StringBuilder();
      while (m.find()) {
        m.appendReplacement(sb, "$foo");
      }
      System.err.println(sb);
    }
    {
      System.err.println("> java.util.regex");
      var p = java.util.regex.Pattern.compile(pattern);
      var m = p.matcher(input);
      StringBuilder sb = new StringBuilder();
      while (m.find()) {
        m.appendReplacement(sb, "$foo");
      }
      System.err.println(sb);
    }

> re2j
$foo$foo

> java.util.regex
java.lang.IllegalArgumentException: Illegal group reference
	at java.base/java.util.regex.Matcher.appendExpandedReplacement(Matcher.java:1111)
	at java.base/java.util.regex.Matcher.appendReplacement(Matcher.java:1041)