 <p>Correct - the positive lookbehinds will not work.</p> <p>But, just as some general information about regex in Javascript, here's a couple pointers for you.</p> <p>You don't have to use the RegExp object - you can use pattern literals instead</p> <pre><code>var regex = /^[a-z\d]+$/i; </code></pre> <p>But if you use the RegExp object, you have to escape your backslashes since your pattern is now locked in a string.</p> <pre><code>var regex = new RegExp( '^[a-z\\d]+$', 'i' ); </code></pre> <p>The primary benefit of the RegExp object is if there is a dynamic bit to your pattern, for example</p> <pre><code>var max = 4; var regex = new RegExp( '\d{1,' + max + '}' ); </code></pre>