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In order to use regular expression operators in MongoDB to modify a string in a JSON file, you can use the $regex operator in combination with other update operators such as $set, $replaceOne, or $replaceAll. Here is an example of how to use these operators:

Suppose you have a collection users in a MongoDB database that contains documents with the following structure:

{
  "_id": "123",
  "name": "John Doe",
  "email": "john.doe@example.com"
}

Now, suppose you want to modify the email address of all users whose email provider is example.com by replacing it with gmail.com.

You can use the $regex operator to match all email addresses that contain the string example.com. Here is an example of how to do this using the $set operator:

db.users.updateMany(
  { email: { $regex: /example\.com/ } },
  { $set: { email: { $regexReplace: { input: "$email", find: "example.com", replace: "gmail.com" } } } }
)

In this example, we use the updateMany method to update all documents that match the $regex expression { email: { $regex: /example\.com/ } }. Then, we use the $set operator to update the email field of each matched document by using the $regexReplace operator.

The $regexReplace operator takes three parameters:

• input: the input string to modify (in this case, the value of the email field)
• find: the string to find (in this case, "example.com")
• replace: the string to replace it with (in this case, "gmail.com")

After running this update operation, all user documents whose original email address contained the string example.com will now have their email address replaced with gmail.com.