Extract substring between two specified substrings in java

Extract substring between two specified substrings in java

I am getting a string in a variable filename like this...
String filename = String.format(getBusinessDate()+ getFileDate() );
and the outcome is as ..
2013081020130810 07:40
Now i want to insert a "_" in between that is between business date and
file date
as shown below , please advise how to achieve this
20130810_20130810 07:40