Regex problem..

Example:

9980767614,8NWZNbd3
1418650724,qlyitjie
28547211,gjipmcno
248872383,nbqnuavc
6688785775,agsyktqq >>>if this return false..
6688785775                >>if this return true

I want to check the numbers only which has 6-12 digits in the txt file..(no other things only 6-12 digits numbers)

If yes, return true..
else return false..

Thanks for replying..

Regard