Regex Length issue - regex

Question

I'm trying to build a regex where it accepts domain names with the following conditions:
Allows DNS names (only hyphens, periods and alphanumeric characters allowed) upto 255 characters.
Hyphens can only appear in between letters
Should start with a letter and end with a letter. It will have minimum 3 characters (letters and periods mandatory, hyphen is optional.)
The length of the label before a period should be 63
Possible Cases:
a.b.c
a-a.b
Cases that should not pass
a-.b
qwertqwertqwertqwertqwertqwertqwertqwertqwertqwertqwertqwertqwerhhg.v
aaaa
aaa-a
What I have built looks like this:
^(([a-zA-z0-9][A-Z0-9a-z-]{1,61}[a-zA-Z0-9][.])+[a-zA-Z0-9]+)$
But this does not accept a.b.c

Answer 1

You may use
^(?=.{1,255}$)(?=[^.]{1,63}(?![^.]))[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*(?:[.](?=[^.]{1,63}(?![^.]))[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*)+(?:[.][a-zA-Z0-9-]*[a-zA-Z0-9])?$
See the regex demo here.
Pattern details
^ - start of string
(?=.{1,255}$) - the whole string should have 1 to 255 chars
(?=[^.]{1,63}(?![^.])) - there must be 1 to 63 chars other than . before the char other than . or end of string
[a-zA-Z0-9]+ - 1 or more alphanumeric chars
(?: - start of a non-capturing group:
- - a hyphen
[a-zA-Z0-9]+ - 1+ alphanumeric chars
)* - zero or more repetitions
(?: - start of a non-capturing group...
[.] - a dot
(?=[^.]{1,63}(?![^.])) - there must be 1 to 63 chars other than . before the char other than . or end of string
[a-zA-Z0-9]+ - 1+ alphanumeric chars
(?:-[a-zA-Z0-9]+)* - 0 or more repetitions of a - followed with 1+ alphanumeric chars
)+ -... 1 or more times
(?: - start of a non-capturing group...
[.] - a dot
[a-zA-Z0-9-]* - 1+ alphanumeric or - chars
[a-zA-Z0-9] - an alphanumeric char (no hyphens at the end)
)? -... 1 or 0 times (it is optional)
$ - end of string.

Answer 2

You can use the following regex:
/^(?=[A-Z])((?:[A-Z\d]|(?<=[A-Z])-(?=[A-Z])){1,63})(?<=[A-Z])(?:\.[A-Z\d]+){1,2}$/im
Details:
^ - Start of the string.
(?=[A-Z]) - Positive lookahead: The whole string must start with a letter.
( - A capturing group - the domain name.
(?: - Start of a non-capturing group, needed due to the following quantifier.
[A-Z\d] - The first alternative: Either a letter or a digit.
| - Or.
(?<=[A-Z])-(?=[A-Z]) - The second alternative: A hyphen, preceded with a letter
and followed with a letter.
) - End of the non-capturing group.
{1,63} - This group (either alternative) must occur up to 63 times.
) - End of the capturing group.
(?<=[A-Z]) - Positive lookbehid: The capturing group just matched (domain name)
must end with a letter.
(?: - A non-capturing group, also needed due to the following quantifier.
\.[A-Z\d]+ - A dot and a sequence of letters or digits.
) - End of the non-capturing group.
{1,2} - This group must occur 1 or 2 times.
$ - End of the string.
You should definitely use i (case insensitive) option and if you check
a number of strings, each in a separate row, also m (multiline) option.
I didn't include any test for the whole length, but you didn't include it either.
I think, the main task here was to show how to match the case your regex failed.