JSON escaping and unescaping that really works, no dependencies.

JSONUtil.java

// BSD License (http://lemurproject.org/galago-license)
package org.lemurproject.galago.utility.json;

public class JSONUtil {
  public static String escape(String input) {
    StringBuilder output = new StringBuilder();

    for(int i=0; i<input.length(); i++) {
      char ch = input.charAt(i);
      int chx = (int) ch;

      // let's not put any nulls in our strings
      assert(chx != 0);

      if(ch == '\n') {
        output.append("\\n");
      } else if(ch == '\t') {
        output.append("\\t");
      } else if(ch == '\r') {
        output.append("\\r");
      } else if(ch == '\\') {
        output.append("\\\\");
      } else if(ch == '"') {
        output.append("\\\"");
      } else if(ch == '\b') {
        output.append("\\b");
      } else if(ch == '\f') {
        output.append("\\f");
      } else if(chx >= 0x10000) {
        assert false : "Java stores as u16, so it should never give us a character that's bigger than 2 bytes. It literally can't.";
      } else if(chx > 127) {
        output.append(String.format("\\u%04x", chx));
      } else {
        output.append(ch);
      }
    }

    return output.toString();
  }

  public static String unescape(String input) {
    StringBuilder builder = new StringBuilder();

    int i = 0;
    while (i < input.length()) {
      char delimiter = input.charAt(i); i++; // consume letter or backslash

      if(delimiter == '\\' && i < input.length()) {

        // consume first after backslash
        char ch = input.charAt(i); i++;

        if(ch == '\\' || ch == '/' || ch == '"' || ch == '\'') {
          builder.append(ch);
        }
        else if(ch == 'n') builder.append('\n');
        else if(ch == 'r') builder.append('\r');
        else if(ch == 't') builder.append('\t');
        else if(ch == 'b') builder.append('\b');
        else if(ch == 'f') builder.append('\f');
        else if(ch == 'u') {

          StringBuilder hex = new StringBuilder();

          // expect 4 digits
          if (i+4 > input.length()) {
            throw new RuntimeException("Not enough unicode digits! ");
          }
          for (char x : input.substring(i, i + 4).toCharArray()) {
            if(!Character.isLetterOrDigit(x)) {
              throw new RuntimeException("Bad character in unicode escape.");
            }
            hex.append(Character.toLowerCase(x));
          }
          i+=4; // consume those four digits.

          int code = Integer.parseInt(hex.toString(), 16);
          builder.append((char) code);
        } else {
          throw new RuntimeException("Illegal escape sequence: \\"+ch);
        }
      } else { // it's not a backslash, or it's the last character.
        builder.append(delimiter);
      }
    }

    return builder.toString();
  }
}

JSONUtilTest.java

package org.lemurproject.galago.utility.json;

import org.junit.Test;

import static org.junit.Assert.*;
import static org.lemurproject.galago.utility.json.JSONUtil.escape;
import static org.lemurproject.galago.utility.json.JSONUtil.unescape;

public class JSONUtilTest {

  @Test
  public void testUnescape() throws Exception {

    String testShort = "Eating a piece of \u03c0 (pi)";
    assertEquals("Eating a piece of \\u03c0 (pi)", escape(testShort));

    String testLong = "I stole this guy from wikipedia: \ud83d\ude02"; // emoji "face with tears of joy"
    assertEquals("I stole this guy from wikipedia: \\ud83d\\ude02", escape(testLong));

    String testQuote = "here it comes \" to wreck the day...";
    assertEquals("here it comes \\\" to wreck the day...", escape(testQuote));
    String testNewline = "here it comes \n to wreck the day...";
    assertEquals("here it comes \\n to wreck the day...", escape(testNewline));
    String testBackslash = "here it comes \\ to wreck the day...";
    assertEquals("here it comes \\\\ to wreck the day...", escape(testBackslash));

    String testAsciiEscapes = "\\\r\n\t\b\f \\hmm\\ \f\b\n\r\\";
    assertEquals(testAsciiEscapes, unescape(escape(testAsciiEscapes)));
    assertEquals(testLong, unescape(escape(testLong)));
    assertEquals(testShort, unescape(escape(testShort)));

  }
}