How to use Foreach Loop Editor to find files(.txt) with user-defined format？

I have a question about SSIS. Could i use expressions and variable in Foreach loop editor to find files(.txt)

with use - defined format？

Format Like below:

extract_sae_vat_p0040996agq5_20121221030456
extract_sae_vat_p0040996agq5_20121222030538
extract_sae_vat_p0040996agq5_20121223030424

Thanks,guys