in reply to Re: (jeffa) Re: A regex I cant get my head around
in thread A regex I cant get my head around

Why not find out for yourself? All you need to do is change your code to something like: 
my $as_string = 'startKEY=jdjhjfg8y4uh449845&finish';
$as_string =~ /KEY=([^&]+)\&/;
print $1, "\n";

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Regardless of whether or not this has to do with GET params, you can still use URI to parse any key=value pairs that are delimited by ampersands: 
use strict;
use URI;

my $uri = URI->new();
$uri->query('startKEY=jdjhjfg8y4uh449845&finish');

my %pair = $uri->query_form();
print $pair{startKEY}, "\n";

[download]

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

In Section Seekers of Perl Wisdom