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A Simple ISINDEX Query&toc - Perl

Having trouble parsing a querystring with Perl? If so, then Jeff's step-by-step guide will save you headaches and have you up and running in no time!

  1. Parsing a Querystring With Perl
  2. A Simple ISINDEX Query
  3. A Simple POST Query
By: Jeff Pinyan
Rating: starstarstarstarstar / 7
December 18, 2002

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We already know the arguments are stored in the @ARGV array, so there is technically no need to parse the $ENV{QUERY_STRING} variable. However, the parsing here will be the simplest we encounter.

I mentioned that some servers (such as Apache 1.3.1 for Win32 systems) will allow space characters (character 32) as well as ampersands (character 38) in between keywords. For this functionality, send a true value (1, for example) as the first argument to the function. Also, your system may expect just a single space between keywords; to allow for multiple spaces (and ampersands, if that option is true), send a true value as the second argument to the function. The final option causes leading and trailing spaces to be removed, and this is achieved by sending a true value as the third argument.

This function also makes adjustments for an ISMAPed image that sends XX,YY as the query string.

While the @ARGV array has shell characters escaped, this function merely decodes the query, and does not escape characters.

# @keywords = isindex_query($amp, $squash, $strip);

sub isindex_query {
my ($amp,$squash,$strip) = @_;
my $str = $ENV{QUERY_STRING};
my @kw;

# handle XX,YY
if ($str =~ /^(\d+),(\d+)$/) {
return ($1,$2);

# change %26 (encoding for ampersand) to a + character
$str =~ s/%26/+/g if $amp;

# squish more than one + into one
$str =~ tr/+//s if $squash;

# remove leading and trailing + signs
$str =~ s/^\++//, $str =~ s/\++$// if $strip;

# split query string by + signs
@kw = split /\+/, $str;

# return decoded keywords
return map url_decode, @kw;

We should define the url_decode() and url_encode() functions right now, too, since they will be used over and over.

# $decoded = url_decode($string);
# $decoded = url_decode;

sub url_decode {
# default argument is $_
local $_ = @_ ? shift : $_;
defined or return;

# change + signs to spaces
tr/+/ /;

# change hex escapes to the proper characters
s/%([a-fA-F0-9]{2})/pack "H2", $1/eg;

return $_;

The URLEncode Routine:

# $encoded = url_encode($string);
# $encoded = url_encode;

sub url_encode {
# default argument is $_
local $_ = @_ ? shift : $_;
defined or return;

# change unsafe characters (except for space) to encoded value
s/[^ a-zA-Z0-9._-!~*'()]/sprintf '%%%02X', ord($1)/eg;

# change spaces to +
tr/ /+/;

return $_;

A GET query

For a GET query, we need to figure out how elements are separated. The simplest method is to split() on & or ; to get the pairs, and then again with = to get at the field and value.

# %kv_pairs = get_query($squash, $strip);

sub get_query {
my ($squash,$strip) = @_;
my $str = $ENV{QUERY_STRING};
my %kv;

# & and ; squishing
$str =~ tr/&;/&/s if $squash;

# leading/trailing & and ; removal
$str =~ s/^[&;]+//, $str =~ s/[&;]+$// if $strip;

# for each k=v pair
for (split /[&;]/, $str) {
# third arg of '2' because $_ might be 'a=b=c'
my ($k,$v) = split /=/, $_, 2;

# don't allow for blank key
next if $k eq "";

# XXX: this only allows one value per key!
$kv{url_decode($k)} = url_decode($v);

return %kv;

As the comment states, this query parser does not allow for multiple values for a key, such as in the query take=box&take=candle&take=sword. There are generally two ways to get around this: make the value of the key in the hash a string of comma-separated strings (or some other character, like NUL (\0)), or an array reference to the values. But there is some difficulty in being sure you choose a character (or sequence of characters) that is not found in the data. So I suggest the array reference method:

# %kv_pairs = get_query($squash, $strip);

sub get_query {
my ($squash,$strip) = @_;
my $str = $ENV{QUERY_STRING};
my %kv;

# ; to & translation
$str =~ tr/;/&/;

# & squishing
$str =~ tr/&//s if $squash;

# leading/trailing & removal
$str =~ s/^&+//, $str =~ s/&+$// if $strip;

# for each k=v pair
for (split /&/, $str) {
# third arg of '2' because $_ might be 'a=b=c'
my ($k,$v) = split /=/, $_, 2;

# don't allow for blank key
next if $k eq "";

($k,$v) = map url_decode, ($k,$v);

if (not exists $kv{$k}) { $kv{$k} = $v }
elsif (not ref $kv{$k}) { $kv{$k} = [ $kv{$k}, $v ] }
else { push @{ $kv{$k} }, $v }

return %kv;

If you are noticing that we need to know when to call which one of these functions, you're thinking ahead. After I show how to parse a simple POST and then a m/fd POST (not file uploads yet -- that's later), then I will show a "multiplexor" -- a function that decides which parser to call.

You'll also notice that the GET parser excludes empty field names. This can be changed, if you like, by removing that line. The final code will have that feature as an option to the parser. Also note that if there is a pair without an = (such as "a=b&foo&c=d") then the value is undef, whereas an = with no value after it (such as "a=b&foo=&c=d") sets the value as the empty string.

>>> More Perl Programming Articles          >>> More By Jeff Pinyan

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