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Regex to verify a number is a prime number.

Posted on 2007-12-06
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Last Modified: 2008-02-01
I have been searching through google to find a regex that allows me to only allow prime numbers but can't seem to find it.

Could someone provide me the regex I need to verify if a number is a prime number? thank you! :)
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Question by:Onthrax
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Expert Comment

by:kebabs
ID: 20419005
Can't be done. I can show you a function for it though. What language are you developing in?
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Author Comment

by:Onthrax
ID: 20419014
That would be great also.. I would need it for asp.net and javascript. :)
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Expert Comment

by:kebabs
ID: 20419064
C# ?

For now, here is the JavaScript version:
<script type="text/javascript">
function IsPrime(n) {
  no = 0;
  for(i = 2; i <= n; i++) {
    for(d = 2; d < i; d++) {
      v = i / d;
      if(v != 1 && Math.floor(v)==v) { no = 1; d = i; }
    }
    if(no != 1) result = i;
    no = 0;
  }
  return (result == n)
}
</script>

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Expert Comment

by:kebabs
ID: 20419073
Hmm... that one doesn't allow IsPrime(1)...

This one fixes that:
<script type="text/javascript">
function IsPrime(n) {
  no = 0;
  if (i < 2) return true;
  for(i = 2; i <= n; i++) {
    for(d = 2; d < i; d++) {
      v = i / d;
      if(v != 1 && Math.floor(v)==v) { no = 1; d = i; }
    }
    if(no != 1) result = i;
    no = 0;
  }
  return (result == n)
}
</script>

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Author Comment

by:Onthrax
ID: 20419086
vb, but I can probably convert a C# one, so that would be fine too..
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Expert Comment

by:kebabs
ID: 20419100
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Accepted Solution

by:
kebabs earned 2000 total points
ID: 20419131
Actually, that JS can be simplified to this (derived from .NET examples in above URL):
<script type="text/javascript">
function CheckIfPrime(n) {
  result = true;
  for (i = 2; i <= Math.sqrt(n); i++)
    if (!(result = n % i != 0)) break;
  return result;
}
</script>

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LVL 13

Author Comment

by:Onthrax
ID: 20419463
Thanks kebabs. Your javascript function worked like a charm. However the vb.net example from the URL you provided is flaud. It confirms numbers are prime which are not. For instance 99 is prime when using the function, which is obviously not since it can be divided by 9.

For future reference to this thread. I converted the C# function to VB and works fine now.

Thnx for the help m8!
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Author Comment

by:Onthrax
ID: 20419468
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Author Comment

by:Onthrax
ID: 20419474
Hmm for some reason it has't attached the code snippet twice.. third attempt..


Dim isPrime As Boolean = True
Dim i As Integer = 2
 
While i <= Math.Sqrt(Currentvalue) AndAlso isPrime
    isPrime = Currentvalue Mod i <> 0
    i += 1
End While

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