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Regular expressions in sed can be used to substitute values in JSON data by identifying the specific pattern of the value to be replaced and replacing it with a new value.

For example, if we have the following JSON data:

{
    "name": "John",
    "age": 35,
    "city": "New York"
}

And we want to replace the value of "age" with a new value of 40, we can use sed and regular expressions in the following way:

sed -i 's/\("age":\) [0-9]\+/\1 40/' file.json

This command will find the pattern "age" followed by any number of digits and replace it with the string "age": 40. The "-i" flag indicates that the file should be updated in place.

Similarly, we can use regular expressions to replace other values in the JSON data. For example, to replace the city with "San Francisco", we can use:

sed -i 's/\("city":\) "[A-Za-z ]\+"/\1 "San Francisco"/' file.json

This command will find the pattern "city" followed by a string of letters and spaces (enclosed in quotes) and replace it with the string "city": "San Francisco".

In general, regular expressions can be used to identify patterns of text within the JSON data and replace them with new values. However, it is important to ensure that the regular expression accurately matches the pattern to be replaced and that the replacement value is properly formatted as JSON.