Herong's Tutorial Notes on Perl - Part B
Dr. Herong Yang, Version 4.11

Common Gateway Interface (CGI)

Part:   1  2  

(Continued from previous part...)

CGI Query String

Query String: The remaining part of the requesting URL string immediately after the "?". Query strings are usually used to pass a list of variable and value pairs with "&" as the pair delimiter, and "=" as the variable and value separator.

In order to avoid confusion, some characters are encoded in query strings. " " is encoded into "+". "+", "&", "=", "%" and other special characters are encoded into a hex number prefixed with "%".

Here is a CGI program to parse query strings, CgiQuery.pl: 

#- CgiQuery.pl
#- Copyright (c) 1996 by Dr. Herong Yang
#
   print "Content-Type: text/html\n\n";
   print "<html><body>\n";
   print "<b>Query String Data:</b><br/>\n";
   $query = $ENV{'QUERY_STRING'};
   @list = split( /\&/, $query);
   foreach (@list) {
      ($var, $val) = split(/=/);
      $val =~ s/\'//g;
      $val =~ s/\+/ /g;
      $val =~ s/%(\w\w)/sprintf("%c", hex($1))/ge;
      print($var, ' = ', $val, "<br/>\n");
   }
   print "</html></body>\n";

Try it with this URL: 

http://localhost/cgi-bin/CgiQuery.pl?Exp=1+%2B+2&Res=3

you will get: 

Query String Data:
Exp = 1 + 2
Res = 3

A Simple CGI Application - Calculator.pl

Here is a simple CGI application, Calculator.pl: 

#- Calculator.pl
#- Copyright (c) 1996 by Dr. Herong Yang
#
   &getValues();
   $res = "";
   $res = eval($exp) if ($exp);
   print "Content-Type: text/html\n\n";
   print "<html><body>\n";
   print "<b>Calculator:</b><br/>\n";
   print "<form action=\"/cgi-bin/Calculator.pl\" methor=\"GET\">\n";
   print "<table border=\"0\">\n";
   print "<tr><td align=\"right\">Expression:</td>\n";
   print "<td><input type=\"text\" name=\"exp\"";
   print   " value=\"$exp\"/></td></tr>\n";
   print "<tr><td align=\"right\">Result:</td>\n";
   print "<td>$res</td></tr>\n";
   print "<tr><td></td>\n";
   print "<td><input type=\"submit\" value=\"Evaluate\"/></td></tr>\n";
   print "</table></form>\n";
   print "</html></body>\n";
   exit;   
sub getValues {
   $query = $ENV{'QUERY_STRING'};
   @list = split( /\&/, $query);
   foreach (@list) {
      ($var, $val) = split(/=/);
      $val =~ s/\'//g;
      $val =~ s/\+/ /g;
      $val =~ s/%(\w\w)/sprintf("%c", hex($1))/ge;
      $exp = $val if ($var =~ /exp/);
   }
}

Enjoy it.

Part:   1  2  

Dr. Herong Yang, updated in 2006
Herong's Tutorial Notes on Perl - Part B - Common Gateway Interface (CGI)