Solved

Parsing form data

Posted on 1998-09-04
1
139 Views
Last Modified: 2013-12-25
I have come across the following piece of code many times,when looking at CGI programs written in Perl. Can anyone explain to me exactly what is going on here?

$form_data =~ s/%([\dA-Fa-f][\dA-Fa-f])/pack ("C", hex ($1))/eg;

cheers
andy
0
Comment
Question by:AndrewThomason
1 Comment
 
LVL 7

Accepted Solution

by:
bjt3 earned 200 total points
ID: 1828531
This perl takes input and modifies it as follows:

look for strings of the form %<hex digit><hex digit> and replace them with the corresponding
ASCII character.  For example %41 is "A".  So,

$form_data = "Here are some letters %41 %42 %43 ...\n";
$form_data =~ s/%([\dA-Fa-f][\dA-Fa-f])/pack ("C", hex ($1))/eg;
print $form_data

prints out

Here are some letters A B C ...

Breaking this down, s/pattern/replace/options allows pattern replacement within a string.
=~ handles assignment.

the first pattern is %[\dA-Fa-f][\dA-Fa-f]  which means "percent sign followed by two hex
digits".   This is done by the two regular expressions [\dA-Fa-f] which match any single
character of 0-9 (from \d) and anything inthe range A-F and a-f.  Hence - any hex digit.

The replacement string is the pack function which takes the matching string (the hex number)
as an argument ($1) and generates the binary ASCII character.  (41 = "A").

The option eg says "evaluate the right side as an expression" and "replace globally across the
entire left string".  So, replace all matching elements based on this expression.

Does that help ?
0

Featured Post

On Demand Webinar - Networking for the Cloud Era

This webinar discusses:
-Common barriers companies experience when moving to the cloud
-How SD-WAN changes the way we look at networks
-Best practices customers should employ moving forward with cloud migration
-What happens behind the scenes of SteelConnect’s one-click button

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

This tutorial will discuss fancy secure registration forms, with AJAX technology support. In this article I assume you already know HTML and some JS. I will write the code using WhizBase Server Pages, so you need to know some basics in WBSP (you mig…
It is becoming increasingly popular to have a front-page slider on a web site. Nearly every TV website,  magazine or online news has one on their site, and even some e-commerce sites have one. Today you can use sliders with Joomla, WordPress or …
Learn the basics of lists in Python. Lists, as their name suggests, are a means for ordering and storing values. : Lists are declared using brackets; for example: t = [1, 2, 3]: Lists may contain a mix of data types; for example: t = ['string', 1, T…
The viewer will learn how to create and use a small PHP class to apply a watermark to an image. This video shows the viewer the setup for the PHP watermark as well as important coding language. Continue to Part 2 to learn the core code used in creat…

685 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question