jeffp% perl string-test 
Enter your regex below (keep it normalish).
(\w{5})-(\d{2,4})\.txt
regex is  : '(\w{5})-(\d{2,4})\.txt'
matched by: 't1nif-19.txt'

jeffp% perl string-test 
Enter your regex below (keep it normalish).
(\w{5})-(\d){3}-\1-\2
regex is  : '(\w{5})-(\d){3}-\1-\2'
matched by: 'rqqxI-754-rqqxI-4'


jeffp% perl reverse-test 
Enter your regex below (keep it normalish).
(\w{5})-(\d{2,4})\.txt
normal  : '(\w{5})-(\d{2,4})\.txt'
reversed: 'txt\.(\d{2,4})-(\w{5})'
normal  : '(\w{5})-(\d{2,4})\.txt'
matchfor: 'Fr4eD-978.txt'
matchrev: 'txt.3582-jEp9c'
matchfor: 'UMVGk-5838.txt'

jeffp% perl reverse-test 
Enter your regex below (keep it normalish).
(\w{5})-(\d){3}-\1-\2
normal  : '(\w{5})-(\d){3}-\1-\2'
reversed: '(\d)-(\w{5})-\1(?:\d){2}-\2'
normal  : '(\w{5})-(?:\d){2}(\d)-\1-\2'
matchfor: '85F7g-464-85F7g-4'
matchrev: '3-n7s1h-349-n7s1h'
matchfor: 'rWwij-705-rWwij-5'