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    <p>The rules you have provided don't actually fully validate credit card numbers, they just check for the correct number of digits.</p> <p>If you really want to validate credit card numbers, you'll need to ensure that the <em>check digit</em> (the last digit) matches the rest of the number according to the <a href="http://en.wikipedia.org/wiki/Luhn_algorithm" rel="nofollow">Luhn algorithm</a> - more info, and working code in Javascript, is <a href="http://evolt.org/node/24700" rel="nofollow">available here</a>. Using the regular expressions you provided, <code>4000000000000000</code> is treated as a valid VISA card number but when you check the <em>check digit</em> as well, it isn't.</p> <p>To explain the regular expressions you provided:</p> <p><code>^</code> means the beginning of the string and <code>$</code> means the end. These must always be used to anchor your regular expressions, unless you want it to match on a substring of a larger string.</p> <p><code>[0-9]</code> means any digit (0 to 9 or anything in between). You could also use <code>\d</code> for this (which one of your other examples does). <code>\d</code> is a special character which means "any digit". There are several other such special characters such as <code>\s</code> which means "any whitespace character".</p> <p><code>{12}</code> means repeat the previous letter or bracketed pattern 12 times (no more, no less).</p> <p><code>(?:</code> just begins a bracketed pattern. The difference between <code>(?:</code> and <code>(</code> is that <code>(?:</code> doesn't capture the pattern for retrieval later (using a back reference). None of these examples use back references so <code>(?:</code> is fine (and is usually more efficient).</p> <p><code>)</code> ends a bracketed pattern. Bracketed patterns are just for putting multiple letters/patterns into a group.</p> <p><code>?</code> means that the previous letter, or bracketed pattern, is <em>optional</em> - it can be present either 0 or 1 times. Incidentally, <code>{0,1}</code> would do the same thing.</p> <p>The rest is pretty self-explanatory, except for:</p> <p><code>[68]</code> which means <em>either</em> 6 <em>or</em> 8. If it had a dash in there, like <code>[6-8]</code>, it would mean 6 <em>to</em> 8, which would include 7 as well. But it doesn't, so it excludes 7.</p>
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