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perl, valid dns name

Posted on 2010-11-24
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Last Modified: 2012-05-10
A valid DNS name has to be (it does not have to have .com etc.)
   1. A host name (label) can start or end with a letter or a number
   2. A host name (label) MUST NOT start or end with a '-' (dash)
   3. A host name (label) MUST NOT consist of all numeric values
   4. A host name (label) can be up to 63 characters

I am new to perl.  Can you write either one or 4 separate regular expression (i prefer multiple regex)?
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Question by:rgbcof
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Author Comment

by:rgbcof
ID: 34209360
And also lower case only.
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Expert Comment

by:santoshmotwani
ID: 34209442
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Accepted Solution

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jmatix earned 333 total points
ID: 34209500
This rule will take care of rule 1,2 and 4

$dns =~ /^([a-z0-9][a-z0-9\-]*){1,63}$/;

This will take care of 3.

$dns =~ /[^\d]/;
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Expert Comment

by:käµfm³d 👽
ID: 34209516
santoshmotwani's example is fine. If you would rather it be separated, then this should be what you need:
[1] & [2]
^[a-zA-Z0-9].*[a-zA-Z0-9]$

[3]
^(?!\d+)$

[4]
^.{1,63}$

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Expert Comment

by:käµfm³d 👽
ID: 34209524
@jmatix

Your first pattern fails rule 2.
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Expert Comment

by:käµfm³d 👽
ID: 34209541
Correction to account for lowercase only:
[1] & [2]
^[a-z0-9].*[a-z0-9]$

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Assisted Solution

by:käµfm³d 👽
käµfm³d   👽 earned 167 total points
ID: 34209565
Sheesh...  I must be tired. Correction to 3 as well. Repost of all:
[1] & [2]
^[a-z0-9].*[a-z0-9]$

[3]
^(?!\d+$)

[4]
^.{1,63}$

Open in new window

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Expert Comment

by:Tintin
ID: 34209583
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Expert Comment

by:jmatix
ID: 34209692
@kaufmed:

I am checking if it starts with an alpha-num. So there is no need to check separately that it does not start with dash(-).

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Expert Comment

by:käµfm³d 👽
ID: 34209712
>>  MUST NOT start or end with a '-' (dash)
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Assisted Solution

by:jmatix
jmatix earned 333 total points
ID: 34209797
Oops! Didn't notice the end part (I should have known better):

$dns =~ /^[a-z\d][a-z\d\-]{0,61}[a-z\d]$/;
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Expert Comment

by:käµfm³d 👽
ID: 34209802
@jmatix

Also, the bound on your "length" is from 1 to Infinity. The quantifier applies to EVERYTHING inside the parentheses.
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Expert Comment

by:käµfm³d 👽
ID: 34209806
NM. You caught it in the new one  :)
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Author Closing Comment

by:rgbcof
ID: 34210049
Gents, the solutions are great.   The Regex library is a good way too, but our systems don't have that library and we can't make the udpate to all of them.
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