If I am understanding your request and you are looking to match "20161228" in filenames that end in ".pdf" here's a regular expression that will help:
{REGEX(Match|BUAB_20161228_1_01_007.pdf|^.*20161228.*\.pdf)}
Of course the actual filename would be replaced by a variable of the current filename as you loop through the files. The regular expression looks for the string "20161228" in the filename of a .pdf file. You can get more descriptive and tighten up the matching if the files always follow the pattern you gave in the example to make sure you don't grab a file by accident just because it's name contains the same string.
i.e. this regular expression: ^[A-Z]{4}_20161228_\d_\d{2}_\d{3}\.pdf
Matches the beginning of the line, followed by a character class of an uppercase letter with an indicator showing we want 4 of the previous type of character (so 4 uppercase characters), followed by a literal underscore, followed by the literal 20161228, an underscore, a digit, an underscore, 2 digits, etc.
Hope this helps.
Oh note that in this example the regex matches lowercase "pdf". VisualCron's regex engine is non-standard in that normally one can indicate an OR by using a pipe symbol. I tried using a group of (pdf|PDF) to match on upper or lowercase pdf but it failed since VisualCron reserves the pipe for the argument separator and the paren for the function closing symbol, even when escaped. I could not seem to find any info on alternate characters to use for this which limits the regex functionality somewhat. I suppose before looping through the files you could do a rename in the directory and ensure all extensions are lowercase or something if necessary.