using java regular expression

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.

What have you tried so far?

Baz
– Baz

2012-09-13 17:10:06 +00:00
Commented Sep 13, 2012 at 17:10 — Baz
– Baz, Commented Sep 13, 2012 at 17:10

Mike Samuel · Accepted Answer · 2012-09-13 17:51:54Z

7

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

Replace all occurrences of "\\" with "\\\\"
Replace all occurrences of "_" with "\\_"
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;
}

edited Sep 13, 2012 at 17:51

answered Sep 13, 2012 at 17:11

Mike Samuel

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2 Comments

Lolly Over a year ago

I tried for the string "this is _test" , encoded string I got is "this_is_\_test" which is excepted. But decoded string is "this is \ test" . Which results in wrong string.

Mike Samuel Over a year ago

@Senthilnathan, Yeah. The decoding is not a simple swap and reverse of the input because there is overlap. Will edit.

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