Can't make regular expression with negative lookaheads work to fix unescaped quotes in JSON

Question

I have some JSON code in the following format:

[
  { "abc ": "d ef", "g": "h i", "jk lm no": "pq", "r st": "uvw xyz" },
  { "!1 2": " 3", "4 ": "5 6 7", " 8 ": "9 abc", "def": "hi "NAME" jk" },
  ...
]

I need to add backslashes in front of quotes in "NAME" to be able to parse this JSON correctly. So I need the above string to look like this:

[
  { "abc ": "d ef", "g": "h i", "jk lm no": "pq", "r st": "uvw xyz" },
  { "!1 2": " 3", "4 ": "5 6 7", " 8 ": "9 abc", "def": "hi \"NAME\" jk" },
  ...
]

I tried using regex to replace (?!({ |": |", ))"(?!( }|: "|, ")) with '\\\\"', but I get:

[
  { \"abc ": \"d ef", \"g": \"h i", \"jk lm no": \"pq", \"r st": \"uvw xyz" },
  { \"!1 2": \" 3", \"4 ": \"5 6 7", \" 8 ": \"9 abc", \"def": \"hi \"NAME\" jk" },
  ...
]

Please help to write a correct regular expression.

Answer 1

Try this regex:

(?<![{,:] )"(?![:,]| })

Description

Demo

http://regex101.com/r/tJ2dG0

---

Discussion

Firstly, I assume that your regex flavor supports lookbehind.

Secondly, how did I find this regex you'd say ? Often, when you build a regular expression, you either build it for matching what you want or you build it for matching what you don't want. I use the latter here.

This is the regex for matching valid double quotes:

(?<=[{,:] )"|"(?=[:,]| })

Demo: http://regex101.com/r/oX4uM5

As you can see in the demo, the regex (let's call it R) doesn't capture invalid quotes. So the regex we are looking for is its (particular) opposite (ie !R). particular because we'll take the opposite of the look(behind|ahead) but not the quote inside R.

So

• (?<=...) becomes (?<!...)
• (?=...) becomes (?!...)
• "|" (read it " OR ") becomes (" AND ") simply "

hence the final regex at the top of this answer.