Re^2: substituting capture groups substrings (?...)

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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Re^3: substituting capture groups substrings (?...)
by Athanasius (Archbishop) on Jul 14, 2015 at 15:22 UTC

Hello 1nickt,

I can’t answer for tye, but I think the following test script is instructive:

Read more... (804 Bytes)

Output:

 1:19 >perl 1307_SoPW.pl
                  Rate    capture lookaround
capture     87642419/s         --       -34%
lookaround 133333333/s        52%         --

 1:20 >
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Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re^3: substituting capture groups substrings (?...)
by AnomalousMonk (Archbishop) on Jul 14, 2015 at 18:00 UTC

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'
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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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"I had a problem and I thought, 'I know, I'll solve it with a regex!' Now I have two problems."

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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