use strict;
my $str = '<a class=\\\'fancybox\\\' href=\\\'http://site.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\\' rel=\\\'group\\\'><img src=\\\'http://site.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\\' width=\\\'400px\\\'></img></a>
<a class=\\\'fancybox\\\' href=\\\'http://site2.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\\' rel=\\\'group\\\'><img src=\\\'http://site2.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\\' width=\\\'400px\\\'></img></a>
<a class=\\\'fancybox\\\' href=\\\'http://site.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\\' rel=\\\'group\\\'><img src=\\\'http://site.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\\' width=\\\'400px\\\'></img></a>';
my $regex = qr~<a\s[^>]*?href\s*=\s*\\'https?://site\.ru/[^>]+>\s*<img.+?</a>~sp;
my $subst = '';
my $result = $str =~ s~$regex~$subst~rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html