I have a set of large files of numbers output by some program, that are in a very wide layout -- ie. there are many numbers listed per line separated by spaces -- which is convenient for the program but not for human inspection.

I wanted to 'wrap' the lines at a convenient place. Whilst the numbers are not sequential, they are ordered and so I chose to wrap them such that all the numbers within each 100 range (eg. nnn100 .. nnn199 ) are on a single line.

To clarify, the input (vastly cut down) looks like this:

105 106 107 108 109 110 111 112 113
1115 1116 1117 1118 1119 1120 1121 1122 1123
12345 12346 12347 12348 12349 12350 12351 12353
[download]

And I decided to wrap it to look like this:

105 106 107 108 109
110 111 112 113
1115 1116 1117 1118 1119 
1120 1121 1122 1123
12345 12346 12347 12348 12349 
12350 12351 12353
[download]

I though to do this by replacing the space between (n1)\d\d and (n1+1)\d\d with a newline. Hence I tried the following regex:

#! perl -slw
use strict;

while( <DATA> ) {
    s[\s(\d+)\d\d\K\s(?=(\d+)\d\d\s)]{
        $1 + 1 == $2 ? "\n" : ' '
    }ge;
    print;
}

__DATA__
105 106 107 108 109 110 111 112 113
1115 1116 1117 1118 1119 1120 1121 1122 1123
12345 12346 12347 12348 12349 12350 12351 12353
[download]

But it does nothing. I've since done the job (with multiple passes due to the fixed-length look-behind limitation) of:

perl -ple"s[(?<=\s(\d{3})\d\d)\s(?=(\d{3})\d\d\s)][$1+1==$2 ? qq[\n] :
+ ' ' ]ge" in > out
[download]

But I still don't understand why the above regex fails?