How do I decode information from HTML forms?

Contributors:

• JimEsten jesten@wdynamic.com

---

When the users clicks the submit button the information is transmitted as a URL encoded string.

This string is in the format:

key=value&key=value&key=value

...where each key (the "name" of your form element) and value (what the user inputs) are separated by the ampersand. The key and the value themselves are separated by an equal sign. Any non alpha-numeric characters get represented by a hexadecimal equivalent preceded by a percent sign.

So, to decode, first separate each key/value pair, then separate the keys from the values, and finally convert any characters that aren't already readable.

Here is a generic perl routine that leaves the contents of the form in an associative array %FORM

sub parse {

 local($name, $value, $pair, $buffer, @pairs);

    # Check for request method and handle appropriately

    if ($ENV{'REQUEST_METHOD'} eq 'GET') {
        @pairs = split(/&/, $ENV{'QUERY_STRING'});
    }
    elsif ($ENV{'REQUEST_METHOD'} eq 'POST') {
        read(STDIN, $buffer, $ENV{'CONTENT_LENGTH'});
        @pairs = split(/&/, $buffer);
    }
    else {
        $Error_Message = "Bad request method ($ENV{'REQUEST_METHOD'}).  Use POST or GET";
        return(0);
    }

    # Convert the data to its original format

    foreach $pair (@pairs) {
        ($name, $value) = split(/=/, $pair);

        $name =~ tr/+/ /;
        $name =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
        $name =~ s/\n//g;
        $value =~ tr/+/ /;
        $value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
        $value =~ s/\n//g;
        $FORM{$name} = $value;
    }
}

This can be used directly in the cgi script, or placed in a separate file and "require" that file in each script that parses a form.

(There are also existing libraries of free code to carry out this and other CGI-related tasks in many programming languages. -TB)

---

Previous | Next | Table of Contents

---