Extract JSON from string in .NET

Question

The input string is mix of some text with valid JSON:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<TITLE>Title</TITLE>

<META http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<META HTTP-EQUIV="Content-language" CONTENT="en">
<META HTTP-EQUIV="keywords" CONTENT="search words">
<META HTTP-EQUIV="Expires" CONTENT="0">

<script SRC="include/datepicker.js" LANGUAGE="JavaScript" TYPE="text/javascript"></script>
<script SRC="include/jsfunctions.js" LANGUAGE="JavaScript" TYPE="text/javascript"></script>

<link REL="stylesheet" TYPE="text/css" HREF="css/datepicker.css">

<script language="javascript" type="text/javascript">
function limitText(limitField, limitCount, limitNum) {
    if (limitField.value.length > limitNum) {
        limitField.value = limitField.value.substring(0, limitNum);
    } else {
        limitCount.value = limitNum - limitField.value.length;
    }
}
</script>
{"List":[{"ID":"175114","Number":"28992"]}

The task is to deserialize the JSON part of it into some object. The string can begin with some text, but it surely contains the valid JSON. I've tried to use JSON validation REGEX, but there was a problem parsing such pattern in .NET.
So in the end I'd wanted to get only:

{
    "List": [{
        "ID": "175114",
        "Number": "28992"
    }]
}

Clarification 1:
There is only single JSON object in whole the messy string, but the text can contain {}(its actually HTML and can contain javascripts with <script> function(){..... )

@AndyKorneyev I've tried to, but it can't deserialize the string properly.. So I must somehow tell the JSON.NET how to parse it. — 0x49D1
– 0x49D1, Commented Nov 1, 2016 at 13:50
Why does the text before and after the JSON portion exist? Can the text contain { and }? If not then the simple solution would be to find the 1st and last bracers and assume that is the start and end of your JSON. Otherwise I'd say you are screwed since you wont be able to tell where the actual JSON starts. — Theo
– Theo, Commented Nov 1, 2016 at 13:59
Maybe there are some restrictions on that text? For example, can it contain "{" character? — Evk
– Evk, Commented Nov 1, 2016 at 13:59
You are screwed, as even {} is valid json and chances are there that your html-containing text contains things that could be an actual json out of context. However you might try to use a DOM-parser to extract your JSON from the HTML, if you have a clue where it is placed. You might be even more screwed, if your JSON is formatted with HTML :X — Sebastian Proske
– Sebastian Proske, Commented Nov 1, 2016 at 14:05

Ralf Bönning · Accepted Answer · 2016-11-01 14:17:43Z

7

You can use this method

    public object ExtractJsonObject(string mixedString)
    {
        for (var i = mixedString.IndexOf('{'); i > -1; i = mixedString.IndexOf('{', i + 1))
        {
            for (var j = mixedString.LastIndexOf('}'); j > -1; j = mixedString.LastIndexOf("}", j -1))
            {
                var jsonProbe = mixedString.Substring(i, j - i + 1);
                try
                {
                    return JsonConvert.DeserializeObject(jsonProbe);
                }
                catch
                {                        
                }
            }
        }
        return null;
    }

The key idea is to search all { and } pairs and probe them, if they contain valid JSON. The first valid JSON occurrence is converted to an object and returned.

answered Nov 1, 2016 at 14:17

Ralf Bönning

15.7k6 gold badges56 silver badges71 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

0x49D1 Over a year ago

I was going to do almost such thing by finding the occurrences of '{' by hand and then to match some properties. The accepted answer seems more proper to me. Thank you for your time and help too!

Ralf Bönning Over a year ago

@0x49D1 - You are welcome. Just an additional thought: It might be better to probe from the longest match to the shortest (I am not sure, if there is a defined order in the regex matches). If there is no fixed order, then a subpart of the JSON might be recognized as valid JSON as well.

Chrille · Accepted Answer · 2019-09-25 10:02:14Z

4

Use regex to find all possible JSON structures:

\{(.|\s)*\}

Regex example

Then iterate all these matches unitil you find a match that will not cause an exception:

JsonConvert.SerializeObject(match);

If you know the format of the JSON structure, use JsonSchema.

edited Sep 25, 2019 at 10:02

answered Nov 1, 2016 at 14:17

Chrille

1,45313 silver badges26 bronze badges

4 Comments

0x49D1 Over a year ago

Thanks! With few refinement that's better solution than magic with substrings!

Cem Mutlu Over a year ago

This regex may be dangerous because it may create catastrophic backtracking.

Michael Sync Over a year ago

Thanks @Chrile It doesnt work for the complex json with the nested structure.

Michael Sync Over a year ago

Can someone add the regx that also works for json array (array of json object)?

Collectives™ on Stack Overflow

Extract JSON from string in .NET

2 Answers 2

2 Comments

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related