How to get substring between two strings in DART?

You can use String.indexOf combined with String.substring:

void main() {
  const str = "the quick brown fox jumps over the lazy dog";
  const start = "quick";
  const end = "over";

  final startIndex = str.indexOf(start);
  final endIndex = str.indexOf(end, startIndex + start.length);

  print(str.substring(startIndex + start.length, endIndex)); // brown fox jumps
}

Note also that the startIndex is inclusive, while the endIndex is exclusive.