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    <p>I am no expert at how regex engines actually work, but I know that some (all?) are greedy and will try to match as much as possible as early as possible. So suppose you have string <code>s</code> you are trying to match, which has no <code>'&lt;'</code> characters. It will first match the <code>[^&lt;]*</code> part of the regex, essentially matching everything from <code>s[0]</code> to <code>s[n-1]</code> (<code>s</code> is zero-indexed and doesn't have c-string subtleties, so this is the entire string). It will then fail on the next element in the pattern (the <code>'&lt;'</code> character). Next, it will backtrack to matching <code>[^&lt;]*</code> from <code>s[0]</code> to <code>s[n-2]</code>, attempt to match <code>'&lt;'</code>, and fail again. This will repeat until it matches a string of length 0 at position 0 (note <code>*</code> matches zero or more repetitions and this final case is zero repetitions). So thus it will determine that starting at position 0 cannot result in a successful match, so it will repeat the above, this time starting the range of matched characters at <code>s[1]</code>, only to fail again after exhausting all such ranges. It will then start at position 2 and so on, until it tries to match at after the last character. Then it will give up. </p> <p><strong>Edit:</strong> Your regex will essentially match any part of the string that ends in <code>&lt;?</code> and does not contain another <code>&lt;</code>, for instance in <code>&lt;&lt;?</code> it matches <code>&lt;?</code> and in <code>ba&lt;abc&lt;?def</code>, it matches <code>abc&lt;?</code>. Some of the other suggestions offered will behave differently. In particular, <code>^[^&lt;]*&lt;\?</code> will not match anything in these two examples. </p>
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    1. POWhy does regex "[^<]*<\\?" exhibit exponential time when the text doesn't have a "<"?
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