Cheatsheet Pages / Regex Cheatsheet
A strict regex for IPv4 addresses — every octet really 0-255
The loose digit-count version happily accepts 999.999.999.999; you need every octet validated.
Patterns use the syntax shared by PCRE / Python / JavaScript; engine differences are flagged in the cheatsheet where they matter. Test any pattern at regex101.com — and pick the matching engine.
((25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(25[0-5]|2[0-4]\d|[01]?\d\d?)
# matches: 192.168.1.1 — rejects 256.1.1.1