Regular expresion with C#

I have a string in my config file, and I want to attach a regular expression with the configuration value so that my program will read the string and regular expression and generate a new string based on the regular expression. Here is an example
Here is my config file
 <Group>
          <Name>__StreamIntake_StreamIntake</Name>
         <RegularExpression></RegularExpression>
        </Group>
        <Group>
          <Name>__ClassIntake_StreamIntake</Name>          
          <RegularExpression></RegularExpression>
        </Group>

StreamIntake and ClassIntake are variables and will be replaced by the data from the database.
for example for the first record from database we got the data for StreamIntake=B2009 and ClassIntake=09BA1
so after the replacement my xml file would look like
<Group>
          <Name>__B2009_B2009</Name>
          <Filter/>
          <RegularExpression></RegularExpression>
        </Group>
        <Group>
          <Name>__09BA1_B2009</Name>
          <Filter/>
          <RegularExpression></RegularExpression>
        </Group>

now my program will load this xml file and read the group name and regular expression and generate a string based on the regular expression.
So for the above xml file I want the string __B_2009 for first group and __BA1_2009 for the second group. My question is what could be the regular expression for the above criteria?
I am unable to hardcode the expected string, thats why I want to add a regular expression there, so if format of new string is changed then we dont need to change the code, we could just change the regular expression and get the expected result. For example currently we are expecting __B_2009  so I have added __B2009_B2009 as group name and regular expression will extract data __B_2009, but in fulture we might expect __B2009 so we could redifine our input data and regular expression in xml config file