I'm working on some custom rules for SpamAssassin and I've come across a question I really don't quite understand and have not yet been able to find the answer to. 
#!/usr/bin/perl

use warnings;
use strict;

while (<DATA>){
  print "Match on line $. with []\n" if (/[htm|asp]/); 
  print "Match on line $. with ()\n" if (/(htm|asp)/); 
}

__DATA__
optout.htm
optout.asp
optout.php


OUTPUT:
Match on line 1 with []
Match on line 1 with ()
Match on line 2 with []
Match on line 2 with ()
Match on line 3 with []

[download]
What I'm not clear on is the difference between the actions of the [ ] and ( ). From the actions of my test script, I believe the (htm|asp) is an 'or' condition (htm or asp). I'm not clear on why the [htm|asp] is matching on the third line of data.

Links to more info appreciated.

Thanks,
Mike

Update: Thanks davido and Enlil. Perfect explanations. Thanks for the links for more info too.

In reply to Regex question by cajun

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
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