Regular Expression for validating DNS label ( host name)

Question

I would like to validate a hostname using only regualr expression.

Host Names (or \'labels\' in DNS jargon) were traditionally defined by RFC 952 and RFC 1123 and m

Answer 1

A revised regex based on comments here and my own reading of RFCs 1035 & 1123:

Ruby: \A(?!-)[a-zA-Z0-9-]{1,63}(? (tests below)

Python: ^(?!-)[a-zA-Z0-9-]{1,63}(? (not tested by me)

Javascript: pattern = /^(?!-)[a-zA-Z0-9-]{1,63}$/g; (based on Tom Lime's answer, not tested by me)

Tests:

tests = [
  ['01010', true],
  ['abc', true],
  ['A0c', true],
  ['A0c-', false],
  ['-A0c', false],
  ['A-0c', true],
  ['o123456701234567012345670123456701234567012345670123456701234567', false],
  ['o12345670123456701234567012345670123456701234567012345670123456', true],
  ['', false],
  ['a', true],
  ['0--0', true],
  ["A0c\nA0c", false]
]

regex = /\A(?!-)[a-zA-Z0-9-]{1,63}(?

Notes:

1. Thanks to Mark Byers for the original code fragment
2. solidsnack points out that RFC 1123 allows all-numeric labels (https://tools.ietf.org/html/rfc1123#page-13)
3. RFC 1035 does not allow zero-length labels (https://tools.ietf.org/html/rfc1035): ::= [ [ ] ]
4. I've added a test specifically for Ruby that ensures a new line is not embedded in the label. This is thanks to notes by ssorallen.
5. This code is available here: https://github.com/Xenapto/domain-label-validation - I'm happy to accept pull requests if you want to update it.