Regex to Match a Port Number
Copy the pattern, paste numbers, and see which are valid ports. Below: how the alternation enforces the 65535 ceiling.
What this pattern matches
It matches an integer in the valid port range 0–65535 and rejects out-of-range numbers like 65536 or 99999.
Why the alternation
Regex can't compare numeric magnitude, so the range is decomposed into digit-length buckets:
6553[0-5], 655[0-2]\d, 65[0-4]\d{2}, 6[0-4]\d{3} — five-digit numbers up to 65535.
[1-5]\d{4} — 10000–59999.
[1-9]\d{0,3} — 1–9999 without leading zeros.
0 — the port zero itself.
Validation
Anchor with ^…$ to validate a lone field instead of scanning text with word boundaries.
Use it in your code
import re
text = "80 443 8080 65535 65536 99999 0 22"
pattern = r'''\b(?:6553[0-5]|655[0-2]\d|65[0-4]\d{2}|6[0-4]\d{3}|[1-5]\d{4}|[1-9]\d{0,3}|0)\b'''
for m in re.finditer(pattern, text):
print(m.group())const text = "80 443 8080 65535 65536 99999 0 22";
const re = new RegExp(String.raw`\b(?:6553[0-5]|655[0-2]\d|65[0-4]\d{2}|6[0-4]\d{3}|[1-5]\d{4}|[1-9]\d{0,3}|0)\b`, "g");
console.log(text.match(re));FAQ
Why not just \d{1,5}?
That would accept 99999, which is not a valid port. The alternation enforces the true 65535 ceiling.
Does it allow port 0?
Yes. Port 0 is technically valid (wildcard). Remove the |0 branch and change [1-9] logic if you want 1-65535 only.
It's easier to check the range in code — why regex?
For validating a single field in a form or config where a numeric parse isn't convenient, a range regex is handy. In code, parsing to int and comparing is clearer.