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    <p>This is one of many situations where lookbehind seems like the obvious tool for the job, but you can't use it. The main problem is that a lookbehind subexpression can't use quantifiers like <code>?</code>, <code>*</code>, <code>+</code>, or <code>{min,max}</code>, so this won't work:</p> <pre><code>preg_match('~(?&lt;!//.*)\bthis\b~', $subject, $match); // nope! </code></pre> <p>In most regex flavors you would have to match the whole line with a regex that doesn't allow <code>//</code> to appear anywhere before the token you're looking for. @Gumbo showed one way to do that, and here's another:</p> <pre><code>preg_match('~^(?:(?!//).)*\bthis\b~', $subject, $match); </code></pre> <p>If your goal is to replace the token with another--for example, replacing <code>this</code> with <code>that</code>--you'll have to capture the stuff before it and plug it back in:</p> <pre><code>preg_replace('~^((?:(?!//).)*)\bthis\b~', '$1that', $subject); </code></pre> <p>However, PHP offers another option: <code>\K</code>, Match Point Reset operator (or as I like to call it, "Poor Man's Lookbehind"). </p> <pre><code>preg_match('~^(?:(?!//).)*\K\bthis\b~', $subject, $match); </code></pre> <p>You match the preceding matter in the usual way, but the <code>\K</code> tells the regex engine to <em>pretend</em> the match really started just before the <code>\bthis\b</code>, not at the beginning of the string, so the value of <code>$match[0]</code> is simply <code>"this"</code>. If you're doing a replace, there's no need to capture the preceding matter and plug it back in:</p> <pre><code>preg_replace('~^(?:(?!//).)*\K\bthis\b~', 'that', $subject); </code></pre> <p><code>\K</code> originated in the Perl regex flavor.</p>
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