naturalsciences has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I have done some mistakes in concatenating some of my files. There are lines where line endings should have been replaced with space. Now I have a large file where there are lines like these.

37 35 37 8 17 16 25 36 37 3840 42 42 42 40

I would be interested in splitting all four number combinations with a space so the entire file would be doubles delimited by spaces.

Seemed easy at the firs look but I got a bit stumped. The idea was read in by lines then

$line =~ s/(/d/d/d/d)/replacement/g

Now I always messed up the replacement part I think - it should be along the lines (pseudo) substr $1,0,1 /s substr $1,2,3 but how the hell to stick it into there syntactically correct I couldn't figure out. Or is it not place for such a thing to stick on one line? Anybody here to help me figure this out.

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Re: substituting capture groups substrings
by choroba (Cardinal) on Jul 14, 2015 at 12:22 UTC
    The trick is to split the numbers already when capturing: 
    $line =~ s/(\d\d)(\d\d)/$1 $2/g;

    [download]

    Update: Note that /d and \d are different.

    لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ
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Re: substituting capture groups substrings (?...)
by tye (Sage) on Jul 14, 2015 at 14:14 UTC
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      Hi tye,

      Can you please explain why you would use 
      s/(?<=[0-9]{2})(?=[0-9]{2})/ /g

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      over 
      s/([0-9]{2})([0-9]{2})/$1 $2/g

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      I know that juxtaposing a look-behind and a look-ahead effectively matches a zero-width point between characters, for which you can then subsitute, as you did with a space. But is there a reason other than coolness to use that construct rather than the more familiar "match/capture/use captures in replacement" construct? Do you do so to avoid the overhead of populating and reading the capture variables?

      The way forward always starts with a minimal test.
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        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'

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        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:  <%-(-(-(-<

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Re: substituting capture groups substrings
by Anonymous Monk on Jul 14, 2015 at 12:21 UTC
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      For future browsers: /d should be \d, and really, unless you intend to allow unicode digits, should probably be [0-9].

      --MidLifeXis

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         For future browsers: /d should be \d, and really, unless you intend to allow unicode digits, should probably be [0-9].

        Thats what //a and //aa are for (since v5.13.10 according to perlver)

        $ perl -Mre=debug -le "print qr/\d/"
Compiling REx "\d"
Final program:
   1: DIGIT (2)
   2: END (0)
stclass DIGIT minlen 1
(?^:\d)
Freeing REx: "\d"

$ perl -Mre=debug -le "print qr/\d/u"
Compiling REx "\d"
Final program:
   1: DIGIT (2)
   2: END (0)
stclass DIGIT minlen 1
(?^u:\d)
Freeing REx: "\d"

$ perl -Mre=debug -le "print qr/\d/a"
Compiling REx "\d"
Final program:
   1: DIGITA (2)
   2: END (0)
stclass DIGITA minlen 1
(?^a:\d)
Freeing REx: "\d"

$ perl -Mre=debug -le "print qr/\d/aa"
Compiling REx "\d"
Final program:
   1: DIGITA (2)
   2: END (0)
stclass DIGITA minlen 1
(?^aa:\d)
Freeing REx: "\d"

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