(-,username,domain1.co.uk)\
(-,username,domain2.co.uk)

awk 'BEGIN {musr="(-,username,[^)]+.co.uk)"} {if ($0~musr) print $0}' netgroup

"(-,username,[^)]+.co.uk)\\$"

"(-,username,[^)]+.co.uk)$"

echo  'BEGIN {musr="(-,username,[^)]+.co.uk)"} {if ($0~musr) print $0}' | a2p.exe 
#!/usr/bin/perl
eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
    if $running_under_some_shell;
                        # this emulates #! processing on NIH machines.
                        # (remove #! line above if indigestible)

eval '$'.$1.'$2;' while $ARGV[0] =~ /^([A-Za-z_0-9]+=)(.*)/ && shift;
                        # process any FOO=bar switches

$, = ' ';               # set output field separator
$\ = "\n";              # set output record separator

$musr = '(-,username,[^)]+.co.uk)';

while (<>) {
    chomp;      # strip record separator
    if ($_ =~ $musr) {
        print $_;
    }
}

$musr = "(-,username,[^)]+.co.uk)\\";
Trailing \ in regex m/(-,username,[^)]+.co.uk)\/ at perlmatch.pl line 18, <> line 1.

$musr = "(-,username,[^)]+.co.uk)$";
Final $ should be \$ or $name at perlmatch.pl line 14, within string
syntax error at perlmatch.pl line 14, near "= "(-,username,[^)]+.co.uk)$""
Execution of perlmatch.pl aborted due to compilation errors.

$musr = "(-,username,[^)]+.co.uk)\$";
[the output is nothing]