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    <p>You're running into <a href="http://www.regular-expressions.info/catastrophic.html" rel="noreferrer">catastrophic backtracking</a>.</p> <p>The reason for this is that you have made the separators optional, and therefore the <code>[^,;]+</code> part (which is itself in a repeating group) of your regex will try loads of permutations (of <code>baaaaaaaz</code>) before finally having to admit failure when confronted with more than two commas.</p> <p><a href="http://www.regexbuddy.com" rel="noreferrer">RegexBuddy</a> aborts the match attempt after 1.000.000 steps of the regex engine with your last test string. Python will keep trying.</p> <p>Imagine the string <code>baaz,,,</code>:</p> <p>Trying your regex, the regex engine has to check all these:</p> <ol> <li><code>baaz,,&lt;failure&gt;</code></li> <li><code>baa</code> + <code>z,,&lt;failure&gt;</code></li> <li><code>ba</code> + <code>az,,&lt;failure&gt;</code></li> <li><code>ba</code> + <code>a</code> + <code>z,,&lt;failure&gt;</code></li> <li><code>b</code> + <code>aaz,,&lt;failure&gt;</code></li> <li><code>b</code> + <code>aa</code> + <code>z,,&lt;failure&gt;</code></li> <li><code>b</code> + <code>a</code> + <code>az,,&lt;failure&gt;</code></li> <li><code>b</code> + <code>a</code> + <code>a</code> +<code>z,,&lt;failure&gt;</code></li> </ol> <p>before declaring overall failure. See how this grows exponentially with each additional character?</p> <p>Behavior like this can be avoided with possessive quantifiers or atomic groups, both of which are sadly not supported by Python's current regex engine. But you can do an inverse check easily:</p> <pre><code>if ",,," in mystring or ";;;" in mystring: fail() </code></pre> <p>without needing a regex at all. If <code>,;,</code> and the likes could also occur and should be excluded, then use Andrew's solution.</p>
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    1. PO100% CPU usage with a regexp depending on input length
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    1. PO100% CPU usage with a regexp depending on input length
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    1. COThe regex implementation on PyPI is much less prone to this kind of problem.
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    2. COThas was a great explanation, it's nice to know the origin of the issue. I think I'll go with the inverse checking for now and discard the regexp. Thanks!!
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