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IP address regex tester

Test IPv4 and IPv6 patterns with strict 0-255 octets, :: compressed forms, IPv4-mapped addresses, CIDR suffixes and private ranges. Pick a preset, edit the regex, paste addresses one per line, then copy JavaScript or an HTML pattern snippet.

Why \d{1,3} is not enough for IPv4

The common shortcut ^(\d{1,3}\.){3}\d{1,3}$ accepts 999.999.999.999. The strict preset spells out each octet range (250-255, 200-249, 100-199, 0-99) and does not allow leading zeros, so 01.2.3.4 fails. Some parsers read a leading zero as octal, which turns 010.0.0.1 into a different address.

IPv4, IPv6 and CIDR patterns

Default preset: ^(?:(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)\.){3}(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)$. Every preset below was checked against its examples with the plain, u and v flags.

HTML IP address input

<label for="ip">Server IP address</label>
<input id="ip" name="ip" type="text" inputmode="decimal" pattern="(?:(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)\.){3}(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)" title="IPv4 address, for example 192.168.1.1" maxlength="15" required>

Use the pattern attribute without ^ or $ (the browser anchors the whole value). Browsers compile it with the v flag, so a - inside a character class must be escaped. Pair it with required and a maxlength (15 for IPv4, 45 for IPv6).

Check again on the server

Regex is fine for form input, but server code should parse the address too. In Node, net.isIP() returns 4, 6 or 0 and handles every valid IPv6 form.

import { isIP } from "node:net";

const IPV4_RE = new RegExp("^(?:(?:25[0-5]|2[0-4]\\d|1\\d\\d|[1-9]?\\d)\\.){3}(?:25[0-5]|2[0-4]\\d|1\\d\\d|[1-9]?\\d)$", "u");
const value = input.trim();
const looksOk = IPV4_RE.test(value);   // quick form check
const family = isIP(value);           // 4, 6 or 0 (invalid): trust this one

$ip = trim($_POST['ip'] ?? '');
$ok = filter_var($ip, FILTER_VALIDATE_IP) !== false;
// IPv4 only: FILTER_VALIDATE_IP, FILTER_FLAG_IPV4
// Public only: add FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE

import ipaddress

try:
    ip = ipaddress.ip_address(value.strip())
    ok, private = True, ip.is_private
except ValueError:
    ok, private = False, False

IP address regex FAQ