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copied!<p>You can try (<a href="http://www.rubular.com/r/SfA309QRWF" rel="noreferrer">as seen on rubular.com</a>)</p> <pre><code>^(?=\d{5}$)1?2?3?4?5?6?7?8?9?0?$ </code></pre> <h3>Explanation</h3> <ul> <li><code>^</code> and <code>$</code> are the beginning and end of string anchors respectively</li> <li><code>\d{5}</code> is the digit character class <code>\d</code> repeated exactly <code>{5}</code> times</li> <li><code>(?=...)</code> is a positive lookahead</li> <li><code>?</code> on each digit makes each optional</li> </ul> <h3>How it works</h3> <ul> <li>First we use lookahead to assert that anchored at the beginning of the string, we can see <code>\d{5}</code> till the end of the string</li> <li>Now that we know that we have 5 digits, we simply match the digits in the order we want, but making each digit optional <ul> <li>The assertion ensures that we have the correct number of digits</li> </ul></li> </ul> <h3>regular-expressions.info</h3> <ul> <li><a href="http://www.regular-expressions.info/anchors.html" rel="noreferrer">Anchors</a>, <a href="http://www.regular-expressions.info/charclass.html" rel="noreferrer">Character Classes</a>, <a href="http://www.regular-expressions.info/repeat.html" rel="noreferrer">Finite Repetition</a>, <a href="http://www.regular-expressions.info/lookaround.html" rel="noreferrer">Lookarounds</a>, and <a href="http://www.regular-expressions.info/optional.html" rel="noreferrer">Optional</a></li> </ul> <hr> <h3>Generalizing the technique</h3> <p>Let's say that we need to match strings that consists of:</p> <ul> <li>between 1-3 vowels <code>[aeiou]</code></li> <li>and the vowels must appear in order</li> </ul> <p>Then the pattern is (<a href="http://www.rubular.com/r/SfA309QRWF" rel="noreferrer">as seen on rubular.com</a>):</p> <pre><code>^(?=[aeiou]{1,3}$)a?e?i?o?u?$ </code></pre> <p>Again, the way it works is that:</p> <ul> <li>Anchored at the beginning of the string, we first assert <code>(?=[aeiou]{1,3}$)</code> <ul> <li>So correct alphabet in the string, and correct length</li> </ul></li> <li>Then we test for each letter, in order, making each optional, until the end of the string</li> </ul> <hr> <h3>Allowing repetition</h3> <p>If each digit can repeat, e.g. <code>11223</code> is a match, then:</p> <ul> <li>instead of <code>?</code> (zero-or-one) on each digit,</li> <li>we use <code>*</code> (zero-or-more repetition) </li> </ul> <p>That is, the pattern is (<a href="http://www.rubular.com/r/GNvWYxJ8hA" rel="noreferrer">as seen on rubular.com</a>):</p> <pre><code>^(?=\d{5}$)1*2*3*4*5*6*7*8*9*0*$ </code></pre>

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