How to extract out the matched line of a txt file?

[markleo]

I have a special content txt file like this,

 
== desc.xxx >>>
11111
2222
333
== desc.xxx <<<
4
555
666
zzz
== desc.yyy >>>
rrr\
== desc.yyy <<<
ttt
iii
ppp
ooo
== desc.zzz >>>
lll
bbhhgfc
 
== desc.zzz <<<
 

my requirement is extract out the matched lines of the text. I want get the below result:

{
  "desc:xxx": {
    "start": 2,
    "end": 6
  },
  "desc.yyy": {
    "start": 11,
    "end": 13
  },
  "desc.zzz": {
    "start": 18,
    "end": 22
  },
  "undefined": [
    {
      "start": 1,
      "end": 1
    },
    {
      "start": 7,
      "end": 10
    },
    {
      "start": 14,
      "end": 17
    }
  ]
}
 

I searched google, but don't find a solution about this.