s/\s*<img.*src="[^"]*(?<!\/someimage)\.jpg".*>//gm

##</code><code>##

s/\s*<img.*src="http:\/\/(?!images\.google\.com\/)[^>]+>//gm

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

my $html_fragment = <<'END_HTML';
    <img src="http://images.google.com/someimage.jpg" />
    <img src="http://images.google.com/NOTsomeimage.jpg" />
    <img src="http://google.somesite.org/image.jpg" />
    <img src="http://somesite.net/google/image.jpg" />
    <img src="http://anythingelse.com/etc.jpg" />
END_HTML

print "Initial markup:\n";
print $html_fragment;

$html_fragment =~ s/\s*<img.*src="[^"]*(?<!\/someimage)\.jpg".*>//gm;

print "Modified markup:\n";
print $html_fragment;

##</code><code>##

Initial markup:
    <img src="http://images.google.com/someimage.jpg" />
    <img src="http://images.google.com/NOTsomeimage.jpg" />
    <img src="http://google.somesite.org/image.jpg" />
    <img src="http://somesite.net/google/image.jpg" />
    <img src="http://anythingelse.com/etc.jpg" />
Modified markup:
    <img src="http://images.google.com/someimage.jpg" />

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

my $html_fragment = <<'END_HTML';
    <img src="http://images.google.com/someimage.jpg" />
    <img src="http://images.google.com/NOTsomeimage.jpg" />
    <img src="http://google.somesite.org/image.jpg" />
    <img src="http://somesite.net/google/image.jpg" />
    <img src="http://anythingelse.com/etc.jpg" />
END_HTML

print "Initial markup:\n";
print $html_fragment;

$html_fragment =~ s/\s*<img.*src="http:\/\/(?!images\.google\.com\/)[^>]+>//gm;

print "Modified markup:\n";
print $html_fragment;

##</code><code>##

Initial markup:
    <img src="http://images.google.com/someimage.jpg" />
    <img src="http://images.google.com/NOTsomeimage.jpg" />
    <img src="http://google.somesite.org/image.jpg" />
    <img src="http://somesite.net/google/image.jpg" />
    <img src="http://anythingelse.com/etc.jpg" />
Modified markup:
    <img src="http://images.google.com/someimage.jpg" />
    <img src="http://images.google.com/NOTsomeimage.jpg" />