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copied!<p><code>?:</code> means do not capture as part of the matching results (aka <strong>capturing group</strong>), basically ignore the stuff in this set of parenthesis.</p> <p>Example:</p> <pre><code>$matches = array(); $card = '4111123456789012'; if (preg_match('/^4[0-9]{12}([0-9]{3})?$/', $card, $matches)) { var_dump($matches); } echo "\n"; </code></pre> <p>Output:</p> <pre><code>array(2) { [0]=> string(16) "4111123456789012" [1]=> string(3) "012" } </code></pre> <p>vs.</p> <pre><code>$matches = array(); $card = '4111123456789012'; if (preg_match('/^4[0-9]{12}(?:[0-9]{3})?$/', $card, $matches)) { var_dump($matches); } echo "\n"; </code></pre> <p>Output:</p> <pre><code>array(1) { [0]=> string(16) "4111123456789012" } </code></pre> <p>Notice in the second example <code>$matches[1]</code> does not exist? That's because you said <code>(?: ...stuff here...)</code> and don't capture it.</p> <p>--</p> <p>In <code>^4[0-9]{12}(?:[0-9]{3})?$</code></p> <p><code>^</code> means the start of the string start matching this pattern.</p> <p><code>4</code> the literal string 4</p> <p><code>[0-9]</code> a single digit of a 0 to 9, inclusive</p> <p><code>{12}</code> is a modifier to the previous character/group. In this case <code>[0-9]{12}</code> means exactly 12 digits</p> <p><code>(?: ... stuff ...)</code> explained above</p> <p><code>?</code> is a modifier to the previous character/group. In this case <code>(...)?</code> zero or 1 of what is in the parenthesis. </p> <p><code>$</code> denotes the end of the matching string.</p> <p>--</p> <p>In <code>^(?:2131|1800|35\d{3})\d{11}$</code></p> <p><code>|</code> denotes OR, so the string can start with 2131, 1800, <strong>OR</strong> 35</p> <p><code>\d</code> is the same as <code>[0-9]</code></p>

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