Matching and storing part of a string in a variable using JScript.NET - regex

Question

I am fiddling with some a script for Fiddler, which uses JScript.NET. I have a string of the format:
{"params":{"key1":"somevalue","key2":"someothervalue","key3":"whatevervalue", ...
I want to match and show "key2":"someothervalue" where someothervalue could be any value but the key is static.
Using good old sed and bash I can replace the part I am looking for with:
$ a='{"params":{"key1":"somevalue","key2":"someothervalue","key3":"whatevervalue", ...'
$ echo $a | sed -r 's/"key2":"[^"]+"/replaced/g'
{"params":{"key1":"somevalue",replaced,"key3":"whatevervalue", ...
Now. Instead of replacing it, I want to extract that part into a variable using JScript.NET. How can that be done?

Answer 1

The most graceful way is to use a JSON parser. My personal preference is to import IE's JSON parser using the htmlfile COM object.
import System;
var str:String = '{"params":{"key1":"foo","key2":"bar","key3":"baz"}}',
htmlfile = new ActiveXObject('htmlfile');
// force htmlfile COM object into IE9 compatibility
htmlfile.IHTMLDocument2_write('<meta http-equiv="x-ua-compatible" content="IE=9" />');
// clone JSON object and methods into familiar syntax
var JSON = htmlfile.parentWindow.JSON,
// deserialize your JSON-formatted string
obj = JSON.parse(str);
// access JSON values as members of a hierarchical object
Console.WriteLine("params.key2 = " + obj.params.key2);
// beautify the JSON
Console.WriteLine(JSON.stringify(obj, null, '\t'));
Compiling, linking, and running results in the following console output:
params.key2 = bar
{
"params": {
"key1": "foo",
"key2": "bar",
"key3": "baz"
}
}
Alternatively, there are also at least a couple of .NET namespaces which provide methods to serialize objects into a JSON string, and to deserialize a JSON string into objects. Can't say I'm a fan, though. The ECMAScript notation of JSON.parse() and JSON.stringify() are certainly a lot easier and profoundly less alien than whatever neckbeard madness is going on at Microsoft.
And while I certainly don't recommend scraping JSON (or any other hierarchical markup if it can be helped) as complicated text, JScript.NET will handle a lot of familiar Javascript methods and objects, including regex objects and regex replacements on strings.
sed syntax:
echo $a | sed -r 's/("key2"):"[^"]*"/\1:"replaced"/g'
JScript.NET syntax:
print(a.replace(/("key2"):"[^"]*"/, '$1:"replaced"'));
JScript.NET, just like JScript and JavaScript, also allows for calling a lambda function for the replacement.
print(
a.replace(
/"(key2)":"([^"]*)"/,
// $0 = full match; $1 = (key2); $2 = ([^"]*)
function($0, $1, $2):String {
var replace:String = $2.toUpperCase();
return '"$1":"' + replace + '"';
}
)
);
... Or to extract the value of key2 using the RegExp object's exec() method:
var extracted:String = /"key2":"([^"]*)"/.exec(a)[1];
print(extracted);
Just be careful with that, though, as retrieving element [1] of the result of exec() will cause an index-out-of-range exception if there is no match. Might either want to if (/"key2":/.test(a)) or add a try...catch. Or better yet, just do what I said earlier and deserialize your JSON into an object.