I have a system that can generate one-level subdomains, I want a regex expression to validate the subdomain before creating it.
So far I have come up with:
^[a-zA-Z0-9]+[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9]*$
from my test cases it
I still am not sure if my pattern is correct. If someone can give me a better regex or point out any rule that I forgot about, please let me know.
There are a couple of problems with your pattern:
Using the + quantifier after the first character class allows for infinitely long strings (the {0,61} quantifier becomes redundant).
Using the * quantifier after the last character class allows it to match zero chars (which makes the pattern accept a hyphen at the end) and also has the same problem as above.
You may use:
^(?=[a-zA-Z0-9].{0,62}$)[a-zA-Z0-9\-]*[a-zA-Z0-9]$
Demo.
Breakdown:
^ - Beginning of the string.(?= - Start of a positive Lookahead.
[a-zA-Z0-9] - The first character must be alphanumeric only..{0,62} - Followed by up to 62 characters.$ - Then, the end of the string.) - End of the Lookahead.[a-zA-Z0-9\-]* - Match 0 or more alphanumeric characters or hyphens.[a-zA-Z0-9]$ - Match one alphanumeric character at the end of the string.If you need to set a minimum length, just change {0,62} to {n,62} where n+1 is the minimum length. Or you could get rid of the Lookahead (as long as the minimum length is >= 2) and use:
^[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9]$
...replacing {0,61} with {n,61} where n+2 is the minimum length.