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POMisunderstanding perl regexp evaluation
 

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    <p>Good time of day! I am reading a book about perl: "Programming Perl" By Larry Wall, Tom Christiansen, Jon Orwant. In this book I found several examples that were not clarified by the authors (or simply I dont get then). </p> <h2>The first</h2> <p>This prints hi only ONCE.</p> <pre><code> "adfsfloglig"=~ /.*(?{print "hi"})f/; </code></pre> <p>But this prints "hi" TWICE?? how can it be explained? </p> <pre><code> "adfsfloglig"=~ /.*(?{print "hi"})log/; </code></pre> <p>And continuing to experement even make things worse: </p> <pre><code> "adfsfloglig"=~ /.*(?{print "hi"})sflog/; </code></pre> <p>The above string of code again prints only ONCE this terrifying "hi" ! After about a week I understood only one thing completely - I NEED HELP :) SO I am asking you to help me, please.</p> <h2>The second (this is a bomb!)</h2> <pre><code> $_ = "lothiernbfj"; m/ (?{$i = 0; print "setting i to 0\n"}) (.(?{ local $i = $i + 1; print "\ti is $i"; print "\tWas founded $&amp;\n" }))* (?{print "\nchecking rollback\n"}) er (?{ $result = $i; print "\nsetting result\n"}) /x; print "final $result\n"; </code></pre> <p>Here the <code>$result</code> finally printing on the screen is equal to number of chars that were matched by <code>.*</code>, but I don't get it again. </p> <p>When turning on debug printing(shown above), i see, that <code>$i</code> is being incremented every time the new char is included in <code>$&amp;</code> (matched part of a string). </p> <p>In the end <code>$i</code> is equal 11 (amount of chars in a string), then there are 7 rollbacks, when <code>.*</code> returns from its match char at a time (7 times) so the match of an all pattern occurs. </p> <p>But, damn magic, the result is setting to value of <code>$i</code>! And we were not decrementing this value anywhere! So <code>$result</code> should be equal 11! But it is not. And authors were right. I know.</p> <p>Please, can you explain this strange perl code, i was happy to met? Thank you for any answer! </p>
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    1. COPerhaps `use re 'debug'` can help cast some light on it.
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    2. COThe "undo" of changes to locals during backtracting is explained [in the docs](http://perldoc.perl.org/perlre.html#(%3f%7b-code-%7d)), hard to summarize more than that.
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