How to get Required character from a string?

Hi,
   i have a value in string str1=a;
Now what i want to do is, have to display all the field values starts with 'a'.
   How to compare?
i tried like this: 
                      string str2="hi,hello,apple,account,board";
                      i have read all those values to an Array. => Array arr = Regex.Split(str2, ",");
           Now how can i pick the matched elements from Array(arr) by comparing with str1, i.e 'a'.
Thanking you.