How do I decode a string with escaped unicode?

Using JSON.decode for this comes with significant drawbacks that you must be aware of:

  • You must wrap the string in double quotes
  • Many characters are not supported and must be escaped themselves. For example, passing any of the following to JSON.decode (after wrapping them in double quotes) will error even though these are all valid: \\n, \n, \\0, a"a
  • It does not support hexadecimal escapes: \\x45
  • It does not support Unicode code point sequences: \\u{045}

There are other caveats as well. Essentially, using JSON.decode for this purpose is a hack and doesn't work the way you might always expect. You should stick with using the JSON library to handle JSON, not for string operations.

I recently ran into this issue myself and wanted a robust decoder, so I ended up writing one myself. It's complete and thoroughly tested and is available here: https://github.com/iansan5653/unraw. It mimics the JavaScript standard as closely as possible.

Explanation:

The source is about 250 lines so I won't include it all here, but essentially it uses the following Regex to find all escape sequences and then parses them using parseInt(string, 16) to decode the base-16 numbers and then String.fromCodePoint(number) to get the corresponding character:

/\\(?:(\\)|x([\s\S]{0,2})|u(\{[^}]*\}?)|u([\s\S]{4})\\u([^{][\s\S]{0,3})|u([\s\S]{0,4})|([0-3]?[0-7]{1,2})|([\s\S])|$)/g

Commented (NOTE: This regex matches all escape sequences, including invalid ones. If the string would throw an error in JS, it throws an error in my library [ie, '\x!!' will error]):

/
\\ # All escape sequences start with a backslash
(?: # Starts a group of 'or' statements
(\\) # If a second backslash is encountered, stop there (it's an escaped slash)
| # or
x([\s\S]{0,2}) # Match valid hexadecimal sequences
| # or
u(\{[^}]*\}?) # Match valid code point sequences
| # or
u([\s\S]{4})\\u([^{][\s\S]{0,3}) # Match surrogate code points which get parsed together
| # or
u([\s\S]{0,4}) # Match non-surrogate Unicode sequences
| # or
([0-3]?[0-7]{1,2}) # Match deprecated octal sequences
| # or
([\s\S]) # Match anything else ('.' doesn't match newlines)
| # or
$ # Match the end of the string
) # End the group of 'or' statements
/g # Match as many instances as there are

Example

Using that library:

import unraw from "unraw";

let step1 = unraw('http\\u00253A\\u00252F\\u00252Fexample.com');
// yields "http%3A%2F%2Fexample.com"
// Then you can use decodeURIComponent to further decode it:
let step2 = decodeURIComponent(step1);
// yields http://example.com

UPDATE: Please note that this is a solution that should apply to older browsers or non-browser platforms, and is kept alive for instructional purposes. Please refer to @radicand 's answer below for a more up to date answer.

This is a unicode, escaped string. First the string was escaped, then encoded with unicode. To convert back to normal: 

var x = "http\\u00253A\\u00252F\\u00252Fexample.com";
var r = /\\u([\d\w]{4})/gi;
x = x.replace(r, function (match, grp) {
    return String.fromCharCode(parseInt(grp, 16)); } );
console.log(x);  // http%3A%2F%2Fexample.com
x = unescape(x);
console.log(x);  // http://example.com

To explain: I use a regular expression to look for \u0025. However, since I need only a part of this string for my replace operation, I use parentheses to isolate the part I'm going to reuse, 0025. This isolated part is called a group.

The gi part at the end of the expression denotes it should match all instances in the string, not just the first one, and that the matching should be case insensitive. This might look unnecessary given the example, but it adds versatility.

Now, to convert from one string to the next, I need to execute some steps on each group of each match, and I can't do that by simply transforming the string. Helpfully, the String.replace operation can accept a function, which will be executed for each match. The return of that function will replace the match itself in the string.

I use the second parameter this function accepts, which is the group I need to use, and transform it to the equivalent utf-8 sequence, then use the built - in unescape function to decode the string to its proper form.


Edit (2017-10-12):

@MechaLynx and @Kevin-Weber note that unescape() is deprecated from non-browser environments and does not exist in TypeScript. decodeURIComponent is a drop-in replacement. For broader compatibility, use the below instead:

decodeURIComponent(JSON.parse('"http\\u00253A\\u00252F\\u00252Fexample.com"'));
> 'http://example.com'

Original answer: 

unescape(JSON.parse('"http\\u00253A\\u00252F\\u00252Fexample.com"'));
> 'http://example.com'

You can offload all the work to JSON.parse


Note that the use of unescape() is deprecated and doesn't work with the TypeScript compiler, for example.

Based on radicand's answer and the comments section below, here's an updated solution:

var string = "http\\u00253A\\u00252F\\u00252Fexample.com";
decodeURIComponent(JSON.parse('"' + string.replace(/\"/g, '\\"') + '"'));

http://example.com

Tags:

Javascript

Decode

Urldecode

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