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PORegex include the negative lookbehind
 

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    <p>I'm trying to filter a string before passing it through <code>eval</code> in python. I want to limit it to math functions, but I'm not sure how to strip it with regex. Consider the following:</p> <pre><code>s = 'math.pi * 8' </code></pre> <p>I want that to basically translate to 'math.pi*8', stripped of spaces. I also want to strip any letters <code>[A-Za-z]</code> that are not followed by <code>math\.</code>.</p> <p>So if <code>s = 'while(1): print "hello"'</code>, I want any executable part of it to be stripped:</p> <p>s would ideally equal something like <code>():""</code> in that scenario (all letters gone, because they were not followed by <code>math\.</code>.</p> <p>Here's the regex I've tried:</p> <pre><code>(?&lt;!math\.)[A-Za-z\s]+ </code></pre> <p>and the python:</p> <pre><code>re.sub(r'(?&lt;!math\.)[A-Za-z\s]+', r'', 'math.pi * 8') </code></pre> <p>But the result is <code>'.p*8'</code>, because <code>math.</code> is not followed by <code>math.</code>, and <code>i</code> is not followed by <code>math.</code>.</p> <p>How can I strip letters that are not in <code>math</code> and are not followed by <code>math.</code>?</p> <h2>What I ended up doing</h2> <p>I followed @Thomas's answer, but also stripped square brackets, spaces, and underscores from the string, in hopes that no python function can be executed other than through the math module:</p> <pre><code>s = re.sub(r'(\[.*?\]|\s+|_)', '', s) s = eval(s, { '__builtins__' : None, 'math' : math }) </code></pre>
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    1. COWhat is the point of this exercise?
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      1. USExplosion Pills
    2. COIf your real purpose is to accept user input that you will then evaluate with `eval`, this is not a very safe approach. [This article](http://lybniz2.sourceforge.net/safeeval.html) provides a safer approach with a good explanation.
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      1. USCarl Groner
    3. COThat's exactly what I'm looking for @Carl
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