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  1. PO
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    <p>(Darn, Jon beat me. Oh well, you can look at the examples anyway)</p> <p>Like the other guys have said, regex is not the best tool for this job. If you are working with filepaths, take a look at <a href="http://docs.python.org/2/library/os.path.html" rel="nofollow">os.path</a>.</p> <p>As for filtering files you don't want, you can do <code>if 'thumb' not in filename: ...</code> once you have dissected the path (where <code>filename</code> is a <code>str</code>).</p> <p>And for posterity, here are my thoughts on those regex. <code>r".*(?!thumb).*"</code> does not work as because <code>.*</code> is greedy and the lookahead is given a very low priority. Take a look at this:</p> <pre><code>&gt;&gt;&gt; re.search('(.*)((?!thumb))(.*)', '/tmp/somewhere/thumb').groups() ('/tmp/somewhere/thumb', '', '') &gt;&gt;&gt; re.search('(.*?)((?!thumb))(.*)', '/tmp/somewhere/thumb').groups() ('', '', '/tmp/somewhere/thumb') &gt;&gt;&gt; re.search('(.*?)((?!thumb))(.*?)', '/tmp/somewhere/thumb').groups() ('', '', '') </code></pre> <p>The last one is quite strange...</p> <p>The other regex (<code>r"^(?!.*thumb).*"</code>) works because <code>.*</code> is inside the lookahead, so you don't have any issues with characters being stolen. You actually don't even need the <code>^</code>, depending on if you are using <code>re.match</code> or <code>re.search</code>:</p> <pre><code>&gt;&gt;&gt; re.search('((?!.*thumb))(.*)', '/tmp/somewhere/thumb').groups() ('', 'humb') &gt;&gt;&gt; re.search('^((?!.*thumb))(.*)', '/tmp/somewhere/thumb').groups() Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; AttributeError: 'NoneType' object has no attribute 'groups' &gt;&gt;&gt; re.match('((?!.*thumb))(.*)', '/tmp/somewhere/thumb').groups() Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; AttributeError: 'NoneType' object has no attribute 'groups' </code></pre>
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    1. POnegative lookahead assertion not working in python
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    1. USBalthamos
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    1. CO+1 for examples
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      1. USJon Clements
    2. CO@Balthamos - in your example I can see very well that the first `(.*)` consumes the complete string and that the `?` makes it non-greedy (why empty? maybe because `*` also allows for 0 matches? not quite an idea). I've tried omitting the `^` and it resulted in `humb.jpg`. So it seems that the (?!.*thumb) consumes everything starting with `thumb`, no matter what comes before it and the trailing `.*`catches everything that does **not** start with `thumb`, namely the rest: `humb.jpg`. Why the negative lookahead does not throw a _no match_ and there is a valid result still remains a mystery to me.
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      1. USErik
    3. COOk, I've tried `re.match` instead of `re.search` and it returned _no match_. It seems the `re.match` does what the `^` did in my working regex example. Good to note that `re.search` is not stopped by a matching negative lookahead if there are other parts of the regex that can be matched (`.*` etc.).
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