first, your input isn't json. Json uses double quotes. But suppose you successfully loaded it with json, it's now a dictionary, called d.

Then you can scan all sub-dicts of d and test serial key against your value, stopping when found using any and a generator comprehension:

print(any(sd['serial']=='00000000762c1d3c' for sd in d['device']))

returns True if serial found False otherwise.

You found a regex that uses syntax that Python standard library re module doesn't support.

When you look at the regex101 link, you'll see that the pattern works when using the PRCE library, and the problematic (?R) syntax that throws the error uses a feature called recursion. That feature is only supported by a subset of regex engines.

You could install the regex library, an alternative regex engine for Python that explicitly does support that syntax:

>>> import regex
>>> pattern = regex.compile(r'\{(?:[^{}]|(?R))*\}')
>>> pattern.findall('''\
... This is a funny text about stuff,
... look at this product {"action":"product","options":{...}}.
... More Text is to come and another JSON string
... {"action":"review","options":{...}}
... ''')
['{"action":"product","options":{...}}', '{"action":"review","options":{...}}']

Another option is to just try and decode any section that starts with { using the JSONDecoder.raw_decode() method; see How do I use the 'json' module to read in one JSON object at a time? for an example approach. While the recursive regex can find JSON-like text, the decoder approach would let you extract only valid JSON text.

Here is a generator function that does just that:

from json import JSONDecoder

def extract_json_objects(text, decoder=JSONDecoder()):
    """Find JSON objects in text, and yield the decoded JSON data

    Does not attempt to look for JSON arrays, text, or other JSON types outside
    of a parent JSON object.

    """
    pos = 0
    while True:
        match = text.find('{', pos)
        if match == -1:
            break
        try:
            result, index = decoder.raw_decode(text[match:])
            yield result
            pos = match + index
        except ValueError:
            pos = match + 1

Demo:

>>> demo_text = """\
This is a funny text about stuff,
look at this product {"action":"product","options":{"foo": "bar"}}.
More Text is to come and another JSON string, neatly delimited by "{" and "}" characters:
{"action":"review","options":{"spam": ["ham", "vikings", "eggs", "spam"]}}
"""
>>> for result in extract_json_objects(demo_text):
...     print(result)
...
{'action': 'product', 'options': {'foo': 'bar'}}
{'action': 'review', 'options': {'spam': ['ham', 'vikings', 'eggs', 'spam']}}

You have to iterate over the list of dictionaries and search for the one with the given id_number. Once you find it you can print the rest of its data and break, assuming id_number is unique.

data = [
 {
   "id_number": "SA4784",
   "name": "Mark",
   "birthdate": None
 },
 {
   "id_number": "V410Z8",
   "name": "Vincent",
   "birthdate": "15/02/1989"
 },
 {
   "id_number": "CZ1094",
   "name": "Paul",
   "birthdate": "27/09/1994"
 }
]

for i in data:
    if i['id_number'] == 'V410Z8':
        print(i['birthdate'])
        print(i['name'])
        break

If you have control over the data structure, a more efficient way would be to use the id_number as a key (again, assuming id_number is unique):

data =  { "SA4784" : {"name": "Mark", "birthdate": None},
          "V410Z8" : { "name": "Vincent", "birthdate": "15/02/1989"},
          "CZ1094" : {"name": "Paul", "birthdate": "27/09/1994"}
        }

Then all you need to do is try to access it directly:

try:
    print(data["V410Z8"]["name"])
except KeyError:
    print("ID doesn't exist")
>> "Vincent"

import json
import sys
import os
data = []
if os.stat("data.txt").st_size != 0 :
    file = open('data.txt', 'r')
    data = json.load(file)
    print(data)

choice = input("What's your choice ?")
if choice == 'a':
    # Add a new joke.
    # See Point 3 of the "Requirements of admin.py" section of the assignment brief.
    jokeSetup = input('Enter setup of joke: ')
    jokePunchLine = input('Enter punchline of joke: ')
    entry = {'setup': jokeSetup , 'punchline': jokePunchLine}
    data.append(entry)
    file = open('data.txt', 'w')
    json.dump(data, file)
    file.close()
    print('Joke Added.')
    pass


elif choice == 's':
    # Search the current jokes.
    # See Point 5 of the "Requirements of admin.py" section of the assignment brief.
    searchTerm = input('Enter search term: ')
    file = open('data.txt', 'r')
    data = json.load(file)
    file.close()

    for sub_dict in data:
        if searchTerm in sub_dict['setup']:
            print(sub_dict['punchline'])
    pass
# or you could modify the last for loop, like this:
    for dict in data:
        if searchTerm in dict['setup'] or searchTerm in dict['punchline']:
            print('found!')
    pass