Is the prefix always "acct"? Is the suffix always "pdf"? Or do you know something a bit more general, such as "the prefix always has letters but no numbers and the suffix always follows a period"?

If it's the former case (always acct and always pdf), then you can simply do this for your pattern:

     acct(\d+)\.pdf

If it's more general (ie prefix can be multiple letters, suffix is any letters or numbers after the period) then you can do something more general:

      [a-zA-Z]+(\d+)\.\w+

I'll explain this in parts:

[a-zA-Z]+   :    One or more of any lowercase or uppercase alphabetical letter. More specifically, letters a through z, and A through Z. 

(\d+)          :   The numbers you want to "capture". \d+ means "one or more digits (ie same as [0-9]+, or one or more characters from 0 to 9)". The parentheses turn this into a "capture", meaning the pattern match will result in just the bit you capture, effectively trimming out the rest of the input.

\.                :  The period, escaped. In a regex, a period character by itself is used to symbolize a wildcard; as in, it can represent any character. By using a slash, "\", you tell the regex you want to match exactly the period character. This slash is very important here!!

\w+           :  One or more word characters. \w matches any alphabetical character as well as any digit. This lets the pattern match suffixes like .pdf but also lets it match suffixes with numbers, like .7z

Oh, I also recommend posting this as a new post here for visibility.

Let me know if you have any questions =)