c:\@Work\Perl\monks>perl -le
"use warnings;
 use strict;
 ;;
 my $s = qq{aaa       : AAA\n}
       . qq{bbb       : BBB\n}
       . qq}ccc       : CCC\n}
       ;
 print qq{[[$s]]};
 ;;
 my $m = 'bbb';
 ;;
 my $t = $s =~
   s/[.\n]*?^$m *: (.*)$[.\n]*/$1/rm
   ;
 ;;
 print qq{[[$t]]};
"
[[aaa       : AAA
bbb       : BBB
ccc       : CCC
]]
[[aaa       : AAA
bbb       : BBB
ccc       : CCC
]]

##</code><code>##

c:\@Work\Perl\monks>perl -le
"use warnings;
 use strict;
 ;;
 my $s = qq{aaa       : AAA\n}
       . qq{bbb       : BBB\n}
       . qq}ccc       : CCC\n}
       ;
 print qq{[[$s]]};
 ;;
 my $m = 'bbb';
 ;;
 my $t = $s =~
   s/[.\n]*?^$m *: (.*)$(?:[.\n]*)/$1/rm
   ;
 ;;
 print qq{[[$t]]};
"
[[aaa       : AAA
bbb       : BBB
ccc       : CCC
]]
[[aaa       : AAABBBccc       : CCC
]]

##</code><code>##

c:\@Work\Perl\monks>perl -le
"use warnings;
 use strict;
 ;;
 my $s = qq{aaa       : AAA\n}
       . qq{bbb       : BBB\n}
       . qq{ccc       : CCC\n}
       ;
 print qq{[[$s]]};
 ;;
 my $m = 'bbb';
 ;;
 my $t = $s =~
   s/ [.\n]*? ^ $m [ ]* : [ ] (.*) $ [.\n]* /$1/xrm
   ;
 ;;
 print qq{[[$t]]};
"
[[aaa       : AAA
bbb       : BBB
ccc       : CCC
]]
[[aaa       : AAABBBccc       : CCC
]]

##</code><code>##

c:\@Work\Perl\monks>perl -wMstrict -e "my $rx = qr{$[.\n]*}m;  print $rx;"
(?^m:0.\n]*)