print lines containing only numbers

Shell Programming and Scripting
1

Hi boys,
I have a txt file with a lot of lines.
It have lines containing mostly only numbers but some of them contain numbers mixed with special characters and or letters or space..
its look like this:

271261621371
2727127f
27126|71372.

...
Do you have any ideas of the easiest way to extract only lines containing numbers? I know its not so complicated but I am stuck on that for an hour.. and still can't find a good way to solve it.
All lines are not equal.. different length. All I need is to grep only lines with ONLY numbers.

Thanks in advance!

2 
egrep -x '[0-9]+'
3

Thanks yazu, egrep works perfect..
I just figured out I should strings file first..
Thank you again !

4