Parsing JSON from a (sub)string

Question

Supposed I have a string that contains JSON:

{"foo":"bar"}

Then I can easily parse this using the JSON.parse function.

Now supposed the string has some additional prefix, such as:

blah: {"foo":"bar"}

Then simply using JSON.parse does not work, of course. What I'd need to do is to extract a substring and then run the function. Now supposed I do not know the content or the length of the prefix (i.e., I don't know whether it finishes with a colon and a space, or whether it is always six characters long, or …): What is the best way to parse the contained JSON?

The most obvious way would probably be to just find the index of the first {, but this won't work if the string itself contains a {.

Is there a better, i.e. more reliable, way to do this?

i think there is no such intelligence provided to us ( or i don't know) ... so that u can remove the unwanted/unpredicted string and find the valid json to parse... the other thing you can do is... search for the substring index of {" and "} ... suppose int indexOfStartOfJson=indexof( {" ); int indexOfEndOfJson=indexof( "} ); and now substring your string ... i.e String validJson=substring(indexOfStartOfJson,indexOfEndOfJson); — Angad Tiwari
– Angad Tiwari, Commented Oct 12, 2015 at 13:49
@BNK This is what I already said I could do, but I was looking for better options ;-) — Golo Roden
– Golo Roden, Commented Oct 12, 2015 at 13:57

Pedro Lobito · Accepted Answer · 2015-10-12 13:55:22Z

2

You may want to use replace to remove the prefix, something like:

test = 'blah: {"foo":"bar"}';
test = test.replace(/^.*?{/, "{");
alert(test);

The above regex works with:

blah: {"foo":"bar"}
blah{"foo":"bar"}
blah:{"foo":"bar"}
blah {"foo":"bar"}

edited Oct 12, 2015 at 13:55

answered Oct 12, 2015 at 13:46

Pedro Lobito

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2 Comments

Golo Roden Over a year ago

Thanks for the snippet, but as said I do not know whether there is always a space in the end, hence this check won't work for all cases.

Pedro Lobito Over a year ago

Just make the space optional, ?. I'll update the regex.

Peter Herdenborg · Accepted Answer · 2015-10-12 13:53:20Z

1

If you don't know anything about the prefix, I guess you might have to remove one character at a time and try parsing the remainder as JSON. As soon as you succeed, you'll have found the starting point.

If the parsing fails a SyntaxError will be thrown which you'll obviously have to handle.

answered Oct 12, 2015 at 13:53

Peter Herdenborg

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2 Comments

Golo Roden Over a year ago

This works, but probably is quite slow… but anyway, thanks for suggesting this!

Peter Herdenborg Over a year ago

Yeah, definitely a last resort, both style wise and performance wise :)

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