I may be reading too much into the OP, but if upaksh wants to translate digit groups in strings like 'f3333_4444_2222' only when the prefix character is an 'f', the other solutions fail (as shown below). Here are a couple of approaches: 

>perl -wMstrict -le
"my %Hidlabels = ('3333' => '1',  '4444' => '2',  '2222' => '3');
 my $t =
   'f3333_4444_2222  g4444_2222_3333  f4444_2222_3333  f3333  _3333';
 ;;
 my $s = $t;
 $s =~ s{ (\d+) }{$Hidlabels{$1}}xmsg;
 print qq{'$s'};
 ;;
 $s = $t;
 $s =~ s{ (?: \G (?<! \A) _ | f) \K  (\d+) }{$Hidlabels{$1}}xmsg;
 print qq{'$s'};
 ;;
 $s = $t;
 my $f = qr{ f \d{4} (?: _ \d{4}){2} }xms;
 $s =~ s{ ($f) }
        { (my $x = $1) =~ s{(\d+)}{$Hidlabels{$1}}xmsg;  $x; }xmsge;
 print qq{'$s'};
"
'f1_2_3  g2_3_1  f2_3_1  f1  _1'
'f1_2_3  g4444_2222_3333  f2_3_1  f1  _3333'
'f1_2_3  g4444_2222_3333  f2_3_1  f3333  _3333'

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In reply to Re: help required for regexp by AnomalousMonk
in thread help required for regexp by upaksh

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