Re: Search and replace regex, but only retain a portion of the string (updated x4)

[AnomalousMonk]

I think I even want to get to UK_Mobile_Vodafone_GBRVF.

Assuming that's really what you want:

c:\@Work\Perl\monks>perl -wMstrict -le
"use constant T => q{UK Mobile - Vodafone [GBRVF] [MSRN]};
 ;;
 my $dest = T;
 $dest =~ s{ \] \s+ \S+ \z }{}xms;
 print qq{2 steps: a: '$dest'};
 ;;
 $dest =~ s{ \W+ }{_}xmsg;
 print qq{2 steps: b: '$dest'};
 ;;
 $dest = T;
 ;;
 $dest =~ s{ \W+ (\w+ \] \z)? }{ $1 ? '' : '_' }xmsge;
 print qq{1 step:     '$dest'};
"
2 steps: a: 'UK Mobile - Vodafone [GBRVF'
2 steps: b: 'UK_Mobile_Vodafone_GBRVF'
1 step:     'UK_Mobile_Vodafone_GBRVF'

Update 1: Or (with inspiration from tybalt89):
    $dest =~ s{ \W+ (\w+ \] \z)? }{ !$1 && '_' }xmsge;

Update 2: Or with no eval:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $dest = q{UK Mobile - Vodafone [GBRVF] [MSRN]};
 ;;
 my @xlate = ('', '_');
 ;;
 $dest =~ s{ \W+ (\w+ \] \z)? }{$xlate[! $1]}xmsg;
 print qq{'$dest'};
"
'UK_Mobile_Vodafone_GBRVF'

With Perl version 5.10+, you could use a persistent state variable:
    state $xlate = [ '', '_' ];
(changing the replacement expression, of course).
(The original code of this update had a  no warnings 'uninitialized'; statement which turned out to be unneeded. Removed.)

Update 3: Or another two-stepper:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $dest = q{UK Mobile - Vodafone [GBRVF] [MSRN]};
 ;;
 $dest =~ tr{A-Za-z}{_}cs;
 $dest =~ s{ _ [^_]+ _ \z }{}xms;
 ;;
 print qq{'$dest'};
"
'UK_Mobile_Vodafone_GBRVF'

Update 4: Or:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $dest = q{UK Mobile - Vodafone [GBRVF] [MSRN]};
 $dest =~ s{ \W+ (\w+ \] \z)? }{ [ '', '_' ]->[! $1] }xmsge;
 print qq{'$dest'};
"
'UK_Mobile_Vodafone_GBRVF'

I'm going to bed now. (Update: I don't know why I based this solution on an anonymous array;  ('', '_')[! $1] is a bit less obscure and probably slightly faster. But beyond that, this eval-ed solution doesn't really offer anything more than the ternary expression used in the eval one-step in my original reply. Well, it was late...)