I would be tempted to go even a little further down the look-around path and specify that the four-digit groups be exactly four digits, i.e., that they be delimited by non-digits. 

c:\@Work\Perl>perl -wMstrict -le
"my $s = '37 35 55555 37 8 17 16 25 36 37 3840 42 42 42 40';
 print qq{'$s'};
 ;;
 $s =~ s{ (?<= (?<! \d) [0-9]{2}) (?= [0-9]{2} (?! \d)) }{ }xmsg;
 print qq{'$s'};
"
'37 35 55555 37 8 17 16 25 36 37 3840 42 42 42 40'
'37 35 55555 37 8 17 16 25 36 37 38 40 42 42 42 40'

[download]

The OPed example data contain an instance of a single-digit data point: '37 8 17'. If I saw such a two-one-two pattern in the output, how would I know (other than by going back and examining the original data) that it was not a regex fix-up artifact? 

c:\@Work\Perl>perl -wMstrict -le
"my $s = '37 35 55555 37 8 17 16 25 36 37 3840 42 42 42 40';
 print qq{'$s'};
 ;;
 $s =~ s{ (?<= [0-9]{2}) (?= [0-9]{2}) }{ }xmsg;
 print qq{'$s'};
"
'37 35 55555 37 8 17 16 25 36 37 3840 42 42 42 40'
'37 35 55 5 55 37 8 17 16 25 36 37 38 40 42 42 42 40'

[download]
Of course, there aren't any five-digit groups in the original data, right? Famous Last Words. This is, at least potentially, an illustration of the classic regex lament "I had a problem and I thought, 'I know, I'll solve it with a regex!' Now I have two problems." Much as I love regexes, I have to admit they can turn nasty on ya pretty quickly. Best to be precise.

Update: Come to think of it, if single-digit data values are valid, who's to say that the proper representation of '3840' is not '3 84 0'? More information is needed to make a useful decision here.

Give a man a fish:  <%-(-(-(-<

In reply to Re^3: substituting capture groups substrings (?...) by AnomalousMonk
in thread substituting capture groups substrings by naturalsciences

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