Question

I am converting a string by replacing all spaces to “_” and if there is “_” in actual string , I am converting it to “\_”. If have a string like “this is test _string” result will be “this_is_test_\_string”, now I want to use java regex to get back original string
“this is test _string”. Is it possible to achieve using java regex ?. Please help me out.

Answer 1

No, it is not possible to get back the original string because you did not escape backslash which makes it ambiguous whether "\\_" came from "_" or "\\ ".

If you had done

1. Replace all occurrences of "\\" with "\\\\"
2. Replace all occurrences of "_" with "\\_"
3. Replace all occurrences of " " with "_"

then you can reverse the process by looking for the tokens "\\\\", "\\_", "_" in a single left to right pass.

In Java, the first transform is

stringToEncode.replace("\\", "\\\\").replace("_", "\\_").replace(" ", "_")

and the dual is

String decode(String stringToDecode) {
  int n = stringToDecode.length();
  StringBuilder out = new StringBuilder(n);
  int decoded = 0;
  for (int i = 0; i < n; ++i) {
    switch (stringToDecode.charAt(i)) {
      case '\\':
        out.append(stringToDecode, decoded, i);
        decoded = ++i;
        break;
      case '_':
        out.append(stringToDecode, decoded, i).append(' ');
        decoded = i+1;
        break;
    }
  }
  return decoded != 0
      ? out.append(stringToDecode, decoded, n).toString()
      : stringToDecode;
}