How to extract substring from string

Hello!

I'm trying extract a substrings from strings to compare with current date. For exemplo:

$str = "teste_20120404_11h00min00seg"

How to extract "20120404" from $str ?

I tried

$str.Split("_") | Select-String [0-9]

but the result is

20120404
11h00min00seg

Thanks!