common replacement code ill-understood

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I see this rather a lot on scripts I download, but I don't really understand it. Can anyone help? Thanks.

$foo =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;

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Re: common replacement code ill-understood by davido (Cardinal) on Jun 11, 2006 at 05:45 UTC
It matches a two-digit hexidecimal number preceded by a %. That entity ( %7F, for example) is converted to a base-10 equivalent (some number between 0 and 255), and then that number is converted into a single byte, and substituted into the original string in place of the original hexidecimal entity. See perlre, hex, and pack for further information. Dave	[reply]
Re: common replacement code ill-understood by Joost (Canon) on Jun 11, 2006 at 06:52 UTC
By the way, that looks an awful lot like a url-decoder. Do yourself a favour and use CGI instead. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply]
Re^2: common replacement code ill-understood by ambrus (Abbot) on Jun 11, 2006 at 09:29 UTC
You can use CGI to decode parameters, but it won't decode the main url part. CGI doesn't have a generic %-unescape routine.	[reply]
Re^3: common replacement code ill-understood by BazB (Priest) on Jun 11, 2006 at 11:12 UTC
To escape and unescape URI/URL escape characters, use URI::Escape. A very handy module. If the information in this post is inaccurate, or just plain wrong, don't just downvote - please post explaining what's wrong. That way everyone learns.	[reply]
Re: common replacement code ill-understood by planetscape (Chancellor) on Jun 11, 2006 at 19:50 UTC
This Tutorial may also be helpful to your understanding: Pack/Unpack Tutorial (aka How the System Stores Data). These are my most frequently used resources for constructing/understanding regexes: Mastering Regular Expressions perlrequick perlretut perlre YAPE::Regex::Explain GraphViz::Regex The Regex Coach (Win32 only, IIRC) HTH, planetscape	[reply]
Re: common replacement code ill-understood by chromatic (Archbishop) on Jun 12, 2006 at 04:40 UTC
It means that the code probably has bugs, especially security problems.	[reply]

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