#!/bin/bash
timestamp_foramt="^.+_INFT_[0-9]{8}_[0-9]{6}\.[[:alnum:]_]+$"
FNAME="Payments.test_INFT_20191218_075918.txt" #example
if [[ ! $FNAME =~ $timestamp_foramt ]]; then
  echo 'non-format';
else
  echo 'format';
fi;