regarding regular expression

[Irfan Sayed]

Hi All,
 
I have a string like this "pqp-un_1.0G_2009-02-23_17-38-09_5678903.txt " now 
what i need is only all digits before .txt till first underscore.
 
which means i need only output as 5678903
 
please please help.
 
Regards
Irfan.