A good IPv4 regex in C#. Should work for most languages with some tweaks.

IPRegex.cs

const string ZERO_TO_255 = "(25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])";
static readonly string IP_REGEX_PATTERN = $@"({ZERO_TO_255}\.){{3}}{ZERO_TO_255}";
static readonly Regex FindIPRegex = new Regex(IP_REGEX_PATTERN, RegexOptions.Compiled);
static readonly Regex ValidateIPRegex = new Regex($"^{IP_REGEX_PATTERN}$", RegexOptions.Compiled);

In JS/TS:

const ZERO_TO_255 = "(25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])";
const IP_REGEX_PATTERN = `(${ZERO_TO_255}\\.){3}${ZERO_TO_255}`;
const ValidateIPRegex = new RegExp(`^${IP_REGEX_PATTERN}$`);