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copied!<p>The first <code>.*</code> initially matches the whole string. Then the regex engine determines whether it needs to back off to match the rest of the regex. But <code>(?<h>((dog)*))</code> and <code>(?(h)(?<dog>(.*)))</code> can both legally match zero characters, so no backtracking is needed (as far as the <code>.*</code> is concerned). Try using a non-greedy <code>.*?</code> in that part.</p> <p><strong>EDIT (in response to the additional info posted in the answer below):</strong> Okay, replacing the first <code>.*</code> with a non-greedy <code>.*?</code> <em>does</em> have an effect, just not the one you want. Where everything after the word "cool" was being captured in group <code><cool></code> before, now it's being captured in group <code><dog></code>. Here's what's happening:</p> <p>After the word "cool" is matched, <code>(?<cool>(.*?))</code> initially matches nothing (the opposite of the greedy behavior), and <code>(?<h>((dog)*))</code> tries to match. This part <em>will always succeed</em> no matter where it's tried, because it can match either "dog" or an empty string. That means the conditional expression in <code>(?(h)...)</code> will always evaluate to <code>true</code>, so it goes ahead and matches the rest of the input with <code>(?<dog>(.*))</code>.</p> <p>As I understand it, you want to match everything after "cool" in named group <code><cool></code>, unless the string contains the word "dog"; then you want to capture everything after "dog" in named group <code><dog></code>. You're trying to use a <a href="http://www.regular-expressions.info/conditional.html" rel="nofollow noreferrer">conditional</a> for that, but it's not really the right tool. Just do this:</p> <pre><code>string pattern = @"cool (?<cool>.*?) (dog (?<dog>.*))?$"; </code></pre> <p>The key here is the <code>$</code> at the end; it forces the non-greedy <code>.*?</code> to keep matching until it reaches the end of the string. Because it's non-greedy, it tries to match the next part of the regex, <code>(dog (?<dog>.*))</code>, before consuming each character. If the word "dog" is there, the rest of the string will be consumed by <code>(?<dog>.*)</code>; if not, the regex still succeeds because the <code>?</code> makes that whole part optional.</p>

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