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Re: pos()atively dangerous.

by dingus (Friar)
on Dec 16, 2002 at 12:06 UTC ( #220180=note: print w/replies, xml ) Need Help??

in reply to pos()atively mysterious.

BTW pos and regex m//g is very dangerous to use in a while loop if you do more regexes on the same string. For example the following code, similar to yours, loops once OK with the source string the same as yours. (The modification is search for a vowel. Search for the next 'x'. Search for next vowel if there is one)

However if we set $var = 'the quick brown fox jumped' then the same code gets into some endless loop whereas $var = 'The quick brown fox jump x' loops twice OK.

$var = 'the quick brown fox'; while( $var =~ m/([aeiou])/g ) { print "\n$var Matched '$1' \@ pos:", pos($var), "\n"; $var =~ m/x/g; print "$var : Matched X \@ pos:", pos($var), "\n"; }
I'm not quite sure whether this is defined behaviour but it caused me some head scratching when trying to parse some text into name/value bits. There are of course any number of work arounds, but its a bit of a surprise when misformed data causes your code to hang.


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Re^2: pos()atively dangerous.
by diotalevi (Canon) on Dec 16, 2002 at 14:15 UTC

    If that matters to you then you just save a copy of the variable's pos value and restore it after stomping on it. This is partly why it's writable.

    $var = 'the quick brown fox'; while( $var =~ m/([aeiou])/g ) { # Save pos $pos = pos $var; # Do something to stomp on $var's pos value # Restore pos pos($var) = $pos; }

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