Regex to match a JSON String

Question

I am building a JSON validator from scratch, but I am quite stuck with the string part. My hope was building a regex which would match the following sequence found on JSON.org:

My regex so far is:

/^\"((?=\\)\\(\"|\/|\\|b|f|n|r|t|u[0-9a-f]{4}))*\"$/

It does match the criteria with a backslash following by a character and an empty string. But I'm not sure how to use the UNICODE part.

Is there a regex to match any UNICODE character expert " or \ or control character? And will it match a newline or horizontal tab?

The last question is because the regex match the string "\t", but not " " (four spaces, but the idea is to be a tab). Otherwise I will need to expand the regex with it, which is not a problem, but my guess is the horizontal tab is a UNICODE character.

Thanks to Jaeger Kor, I now have the following regex:

/^\"((?=\\)\\(\"|\/|\\|b|f|n|r|t|u[0-9a-f]{4})|[^\\"]*)*\"$/

It appears to be correct, but is there any way to check for control characters or is this unneeded as they appear on the non-printable characters on regular-expressions.info? The input to validate is always text from a textarea.

Update: the regex is as following in case anyone needs it:

/^("(((?=\\)\\(["\\\/bfnrt]|u[0-9a-fA-F]{4}))|[^"\\\0-\x1F\x7F]+)*")$/

The above regular expression suffers from inefficiency and ambiguity which can lead to malicious user performing a Denial of Service ("DoS") attack. Here is version that is free of the inefficiency: /^("(((?=\\)\\(["\\\/bfnrt]|u[0-9a-fA-F]{4}))|[^"\\\x00-\x1F\x7F])*")$/ — Vladimír Gorej
– Vladimír Gorej, Commented Jan 25, 2023 at 13:10
@VladimírGorej Further, one of the regex capture groups is unnecessary; /^("((?=\\)\\(["\\\/bfnrt]|u[0-9a-fA-F]{4})|[^"\\\x00-\x1F\x7F])*")$/ is enough. — Ivan Shatsky
– Ivan Shatsky, Commented Dec 25, 2024 at 1:59

InSync · Accepted Answer · 2023-04-18 09:06:53Z

17

For your exact question create a character class

# Matches any character that isn't a \ or "
/[^\\"]/

And then you can just add * on the end to get 0 or unlimited number of them or alternatively 1 or an unlimited number with +

/[^\\"]*/

or

/[^\\"]+/

Also there is this below, found at https://regex101.com/ under the library tab when searching for json

/(?(DEFINE)
# Note that everything is atomic, JSON does not need backtracking if it's valid
# and this prevents catastrophic backtracking
(?<json>(?>\s*(?&object)\s*|\s*(?&array)\s*))
(?<object>(?>\{\s*(?>(?&pair)(?>\s*,\s*(?&pair))*)?\s*\}))
(?<pair>(?>(?&STRING)\s*:\s*(?&value)))
(?<array>(?>\[\s*(?>(?&value)(?>\s*,\s*(?&value))*)?\s*\]))
(?<value>(?>true|false|null|(?&STRING)|(?&NUMBER)|(?&object)|(?&array)))
(?<STRING>(?>"(?>\\(?>["\\\/bfnrt]|u[a-fA-F0-9]{4})|[^"\\\0-\x1F\x7F]+)*"))
(?<NUMBER>(?>-?(?>0|[1-9][0-9]*)(?>\.[0-9]+)?(?>[eE][+-]?[0-9]+)?))
)
\A(?&json)\z/x

This should match any valid json, you can also test it at the website above

EDIT:

Link to the regex

edited Apr 18, 2023 at 9:06

InSync

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answered Aug 22, 2015 at 11:48

user3496378

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4 Comments

Sietse Over a year ago

Thanks for your quick response. I added it to my first regular expression and it seems to be working fine. I don't know anything about the control characters, but maybe I don't need to worry about it as the input is from a textarea where they might not be accepted in. The last regex you provided was a complete regex, but I want to know where the error is. But than again, I'll check it if it might be more useful!

Sietse Over a year ago

I have been playing with your latest regex, and when splitting them, they work great! Thanks!

vbrajon Over a year ago

Thx ! The search url is: regex101.com/library?orderBy=MOST_POINTS&search=json

jcollum Over a year ago

if you could post a code snippet that uses this regex it would be helpful, I got a lot of syntax errors when I pasted that into my code

dazzafact · Accepted Answer · 2022-09-11 15:22:06Z

5

Use this, works also with array jsons [{...},{...}]:

((\[[^\}]{3,})?\{s*[^\}\{]{3,}?:.*\}([^\{]+\])?)

Demo: https://regex101.com/r/aHAnJL/1

answered Sep 11, 2022 at 15:22

dazzafact

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