Apparently this is an actual JSON validator, written in late-model Regexp, using the Ruby interpreter.

is_json.rb

# encoding: utf-8

module Constants
  JSON_VALIDATOR_RE = /(
         # define subtypes and build up the json syntax, BNF-grammar-style
         # The {0} is a hack to simply define them as named groups here but not match on them yet
         # I added some atomic grouping to prevent catastrophic backtracking on invalid inputs
         (?<number>  -?(?=[1-9]|0(?!\d))\d+(\.\d+)?([eE][+-]?\d+)?){0}
         (?<boolean> true | false | null ){0}
         (?<string>  " (?>[^"\\\\]* | \\\\ ["\\\\bfnrt\/] | \\\\ u [0-9a-f]{4} )* " ){0}
         (?<array>   \[ (?> \g<json> (?: , \g<json> )* )? \s* \] ){0}
         (?<pair>    \s* \g<string> \s* : \g<json> ){0}
         (?<object>  \{ (?> \g<pair> (?: , \g<pair> )* )? \s* \} ){0}
         (?<json>    \s* (?> \g<number> | \g<boolean> | \g<string> | \g<array> | \g<object> ) \s* ){0}
       )
    \A \g<json> \Z
    /uix
end

########## inline test running
if __FILE__==$PROGRAM_NAME

  # support
  class String
    def unindent
      gsub(/^#{scan(/^(?!\n)\s*/).min_by{|l|l.length}}/u, "")
    end
  end

  require 'test/unit' unless defined? Test::Unit
  class JsonValidationTest < Test::Unit::TestCase
    include Constants

    def setup

    end

    def test_json_validator_simple_string
      assert_not_nil %s[ {"somedata": 5 }].match(JSON_VALIDATOR_RE)
    end

    def test_json_validator_deep_string
      long_json = <<-JSON.unindent
      {
          "glossary": {
              "title": "example glossary",
          "GlossDiv": {
                  "id": 1918723,
                  "boolean": true,
                  "title": "S",
            "GlossList": {
                      "GlossEntry": {
                          "ID": "SGML",
                "SortAs": "SGML",
                "GlossTerm": "Standard Generalized Markup Language",
                "Acronym": "SGML",
                "Abbrev": "ISO 8879:1986",
                "GlossDef": {
                              "para": "A meta-markup language, used to create markup languages such as DocBook.",
                  "GlossSeeAlso": ["GML", "XML"]
                          },
                "GlossSee": "markup"
                      }
                  }
              }
          }
      }
      JSON

      assert_not_nil long_json.match(JSON_VALIDATOR_RE)
    end

  end
end