Image files
Match names ending in .jpg, .jpeg, .png, or .webp, ignoring letter case.
Challenges
After the 20 levels
Use these open challenges to test combinations of concepts without automatic hints.
Password format
Validate 8 to 16 alphanumeric characters with underscores allowed.
Simple IPv4
Match four numeric blocks separated by dots. First validate the format; then think about the 0–255 range.