I have tried using same json file document mentioned by you, the only change that i did is iterating the Json String. Please see the below code.

It works.

   try{

        List<Document> docList = [Select Id,Type,body, bodyLength, ContentType, Url from Document WHERE ID='01528000003FqHJAA0'];

        String jsonStr='';
        if(docList.size()>0)
        {
            Blob b =  docList[0].body;
            jsonStr = b.toString();  
        }

        Map<String,Object> mapObj = (Map<String,Object>)JSON.deserializeUntyped(jsonStr);
        for (String key : mapObj.keySet()){
            System.debug('KEY :' + key);
            System.debug('Value :' + mapObj.get(key));
        }

    }catch(Exception exe){
        System.debug('EEROR OCCURED : '+exe.getMessage());
    }

It seems there is an error on this line:

"listOfBusinessdays":"1","2","5","6","7"],

A closed bracket square but no open bracket before. May be this hang up the parser.

At the target you can receive the data as text/plain.

Clean it by running :

input.replaceAll("\\p{Cc}", "").

Convert it back to JSON data using any JSON library :

JSONObject inputParams = JSONObject.fromObject(input);

Hope it helps.

I would use a library to create your JSON String for you. Some options are:

• GSON
• Crockford's lib

This will make dealing with escaping much easier. An example (using org.json) would be:

JSONObject obj = new JSONObject();

obj.put("id", userID);
obj.put("type", methoden);
obj.put("msg", msget);

// etc.

final String json = obj.toString(); // <-- JSON string

My solution was this: https://stackoverflow.com/a/40558081/370709

function escapeUnicode(str) {
    return str.replace(/[^\0-~]/g, function(ch) {
        return "\\u" + ("0000" + ch.charCodeAt().toString(16)).slice(-4);
    });
}

Problem solved!

Classes

Here are two classes that solve initial problem with broken json:

public class FixedJson {

    private final String target;

    private final Pattern pattern;

    public FixedJson(String target) {
        this(
            target,
            Pattern.compile("\"(.+?)\"[^\\w\"]")
        );
    }

    public FixedJson(String target, Pattern pattern) {
        this.target = target;
        this.pattern = pattern;
    }

    public String value() {
        return this.pattern.matcher(this.target).replaceAll(
            matchResult -> {
                StringBuilder sb = new StringBuilder();
                sb.append(
                    matchResult.group(),
                    0,
                    matchResult.start(1) - matchResult.start(0)
                );
                sb.append(
                    new Escaped(
                        new Escaped(matchResult.group(1)).value()
                    ).value()
                );
                sb.append(
                    matchResult.group().substring(
                        matchResult.group().length() - (matchResult.end(0) - matchResult.end(1))
                    )
                );
                return sb.toString();
            }
        );
    }
}

public class Escaped {

    private final String target;

    private final Pattern pattern;

    public Escaped(String target) {
        this(
            target,
            Pattern.compile("[\\\\\"]")
        );
    }

    public Escaped(String target, Pattern pattern) {
        this.target = target;
        this.pattern = pattern;
    }

    public String value() {
        return this.pattern
            .matcher(this.target)
            .replaceAll("\\\\$0");
    }
}

Unit-tests

And I've written unit-tests to prove the correctness:

import java.util.regex.Pattern;
import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;

public class FixedJsonTest {

    @Test
    public void normalValue() {
        assertEquals(
            "{\"notes_text\": \"someString\"}",
            new FixedJson("{\"notes_text\": \"someString\"}").value()
        );
    }

    @Test
    public void valueWithOneDoubleQuotes() {
        assertEquals(
            "{\"notes_text\": \"\\\"\"}",
            new FixedJson("{\"notes_text\": \"\"\"}").value()
        );
    }

    @Test
    public void valueWithOneDoubleQuotesAndAnotherValue() {
        assertEquals(
            "{\"notes_text\": \"\\\"\", \"hello\": \"world\"}",
            new FixedJson("{\"notes_text\": \"\"\", \"hello\": \"world\"}").value()
        );
    }

    @Test
    public void valueWithTwoDoubleQuotes() {
        assertEquals(
            "{\"notes_text\": \"\\\"\\\"\"}",
            new FixedJson("{\"notes_text\": \"\"\"\"}").value()
        );
    }

    @Test
    public void valueWithTwoDoubleQuotesAndAnotherValue() {
        assertEquals(
            "{\"notes_text\": \"\\\"\\\"\", \"hello\": \"world\"}",
            new FixedJson("{\"notes_text\": \"\"\"\", \"hello\": \"world\"}").value()
        );
    }

    @Test
    public void valueWithOneSlash() {
        assertEquals(
            "{\"notes_text\": \"\\\\\"}",
            new FixedJson("{\"notes_text\": \"\\\"}").value()
        );
    }

    @Test
    public void valueWithOneSlashAndAnotherValue() {
        assertEquals(
            "{\"notes_text\": \"\\\\\", \"hello\": \"world\"}",
            new FixedJson("{\"notes_text\": \"\\\", \"hello\": \"world\"}").value()
        );
    }

    @Test
    public void valueWithTwoSlashes() {
        assertEquals(
            "{\"notes_text\": \"\\\\\\\\\"}",
            new FixedJson("{\"notes_text\": \"\\\\\"}").value()
        );
    }

    @Test
    public void valueWithTwoSlashesAndAnotherValue() {
        assertEquals(
            "{\"notes_text\": \"\\\\\\\\\", \"hello\": \"world\"}",
            new FixedJson("{\"notes_text\": \"\\\\\", \"hello\": \"world\"}").value()
        );
    }
}

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;

public class EscapedTest {

    @Test
    public void doubleQuotesTest() {
        assertEquals(
            new Escaped("\"").value(),
            "\\\""
        );
        assertEquals(
            new Escaped("abc\"def").value(),
            "abc\\\"def"
        );
    }

    @Test
    public void slashesTest() {
        assertEquals(
            new Escaped("\\").value(),
            "\\\\"
        );
        assertEquals(
            new Escaped("abc\\def").value(),
            "abc\\\\def"
        );
    }

    @Test
    public void mixedTest() {
        assertEquals(
            new Escaped("\\\"").value(),
            "\\\\\\\""
        );
        assertEquals(
            new Escaped("abc\\\"def").value(),
            "abc\\\\\\\"def"
        );
    }
}

Full working example

import java.util.regex.Pattern;

public class FixedJsonExample {
    public static void main(String[] args) {
        String invalidJson = "{\"notes_text\":\"\"\"}";

        final String fixedJson = new FixedJson(invalidJson).value();
        System.out.println("fixedJson = " + fixedJson);
    }


    public static class FixedJson {

        private final String target;

        private final Pattern pattern;

        public FixedJson(String target) {
            this(
                target,
                Pattern.compile("\"(.+?)\"[^\\w\"]")
            );
        }

        public FixedJson(String target, Pattern pattern) {
            this.target = target;
            this.pattern = pattern;
        }

        public String value() {
            return this.pattern.matcher(this.target).replaceAll(
                matchResult -> {
                    StringBuilder sb = new StringBuilder();
                    sb.append(
                        matchResult.group(),
                        0,
                        matchResult.start(1) - matchResult.start(0)
                    );
                    sb.append(
                        new Escaped(
                            new Escaped(matchResult.group(1)).value()
                        ).value()
                    );
                    sb.append(
                        matchResult.group().substring(
                            matchResult.group().length() - (matchResult.end(0) - matchResult.end(1))
                        )
                    );
                    return sb.toString();
                }
            );
        }
    }

    public static class Escaped {

        private final String target;

        private final Pattern pattern;

        public Escaped(String target) {
            this(
                target,
                Pattern.compile("[\\\\\"]")
            );
        }

        public Escaped(String target, Pattern pattern) {
            this.target = target;
            this.pattern = pattern;
        }

        public String value() {
            return this.pattern
                .matcher(this.target)
                .replaceAll("\\\\$0");
        }
    }
}

And its STDOUT:

fixedJson = {"notes_text":"\""}

This JSON can be validated using following tool: https://jsonlint.com/

Regex explanation

FixedJson class

The first class FixedJson uses regexp for matching JSON lemmas: everything between double quotes (include misplaced double quotes).

For more details see an interactive example here: https://regexr.com/54blf

Escaped class

The second class Escaped uses regexp for matching slashes or double quotes. It's required for their escaping.

For more details see an interactive example here: https://regexr.com/54bm1

and the result String is : {\r\n \"xUserPW\": \"dStT0T\"\r\n}

This is not a valid json format. Is webservice returning you json? You cannot simply append \n to received data and convert it.

HttpResponse getResponse =  httpclient.execute(httpPost);  
    HttpEntity returnEntity = getResponse.getEntity();
    is = returnEntity.getContent();
    BufferedReader reader = new BufferedReader(new InputStreamReader(is, "UTF-8"), 128);
    StringBuilder sb = new StringBuilder();

    String line = null;
    while ((line = reader.readLine()) != null)
    {
        sb.append(line);
    }
    JSONObject jObject = new JSONObject(sb.toString());

should not thow errors

Given that

I want ... to get [the] Date(1440959400000+0530) part,

I would use

String value = "/Date(1440959400000+0530)/";
int pos1 = value.indexOf("Date(");
if (pos1 > -1) {
  int pos2 = value.indexOf(")", pos1);
  if (pos2 > pos1) {
    value = value.substring(pos1, pos2 + 1);
    System.out.println(value);
  }
}

Output is

Date(1440959400000+0530)

Note: This works by looking for "Date(" and then the next ")", and it removes everything not between those two patterns.