extract string from filename

[Zhao Peng]

Hi all,

I'm back, with another "extract string" question. //grin

I have almost 1k small files within one folder. The only pattern of the 
file names is:

string1_string2_string3_string4.sas7bdat

Note:
1, string2 often repeat itself across each file name
For example:
abc_st_nh_num.sas7bdat
abc_st_vt_num.sas7bdat
abc_st_ma_num.sas7bdat
abcd_region_NewEngland_num.sas7bdat
abcd_region_South_num.sas7bdat

2, All 4 strings contain no underscores.
3, 4 strings are separated by  3 underscores (as you can see)
4, The length of all 4 strings are not fixed.

My goal is to :
1, extract string2 from each file name
2, then sort them and keep only unique ones
3, then output them to a .txt file. (one unique string2 per line)

I tried to use "cut" commands, but can't even figure out how to use the 
filenames as input. Anyone care to offer me a hint?

I also downloaded an e-book called "Learning Perl" (OReilly, 
4th.Edition), and had a quick look thru its Contents of Table, but did 
not find any chapter which looks likely addressing any issue related to 
my question.

Thank you very much!

Zhao