in reply to Help in joining these lines

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1171851

use strict;
use warnings;

$_ = join '', <DATA>;

1 while s/ ^\w*\K\n(?=\w*\n) | ^.*\..*\K\n(?=.*\.) //mx;

print;

__DATA__
> gi|11SB_CUCMA Train|1 21
MARSSLFTFLCLAVFINGCLSQIEQQSPWEFQGS
EVWQQHRYQSPRACRLENLRAQDPVRLLLPGFSNAPKLIFV
AQGFGIRGIAIPGCAETYQT
SSSSSSSSSSSSSSSSSSSSS....................
...........................
.................
..........
> gi|1A43_HUMAN Train|1 24
MAVMAPRTLVLLLSGALALTQTWAGSHSMRYFYTSVSRPG
RGEPRFIAVGYVDDTQFVRFDSDAASQRMEPRAPWIEQEGPEYWSQTDRANLGTLRGYYNQSEDGSHTIQ
+R
MYGCDVGPDGRFLRGYQQDAYDGKDYIALNEDLRSWTAADMAAQITQRKWETAHEAE
SSSSSSSSSSSSSSSSSSSSSSSS..............................................
+...........................................
.............
..........................................

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Re^2: Help in joining these lines
by Anonymous Monk on Sep 15, 2016 at 15:48 UTC
    Thanks to both of you who helped me!
    Is it possible to explain the pattern match? I am really interested in learning this technique but I am afraid I can't really understand the expression that gods wrote here..
[reply]

      Although use re 'debug'; is available, YAPE::Regex::Explain is a fair amount more detailed in its output when explaining a regex:

      use warnings;
use strict;

use YAPE::Regex::Explain;

my $re = 's/ ^\w*\K\n(?=\w*\n) | ^.*\..*\K\n(?=.*\.) //mx';

print YAPE::Regex::Explain->new($re)->explain;

      [download]

      Output:

      The regular expression:

(?-imsx:s/ ^\w*\K\n(?=\w*\n) | ^.*\..*\K\n(?=.*\.) //mx)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  s/                       's/ '
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  \w*                      word characters (a-z, A-Z, 0-9, _) (0 or
                           more times (matching the most amount
                           possible))
----------------------------------------------------------------------
  \K                       'K'
----------------------------------------------------------------------
  \n                       '\n' (newline)
----------------------------------------------------------------------
  (?=                      look ahead to see if there is:
----------------------------------------------------------------------
    \w*                      word characters (a-z, A-Z, 0-9, _) (0 or
                             more times (matching the most amount
                             possible))
----------------------------------------------------------------------
    \n                       '\n' (newline)
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------
                           ' '
----------------------------------------------------------------------
 |                        OR
----------------------------------------------------------------------
                           ' '
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  .*                       any character except \n (0 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
  .*                       any character except \n (0 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  \K                       'K'
----------------------------------------------------------------------
  \n                       '\n' (newline)
----------------------------------------------------------------------
  (?=                      look ahead to see if there is:
----------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
----------------------------------------------------------------------
    \.                       '.'
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------
   //mx                    ' //mx'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

      [download]

      For the items that still may not be clear, see perlre.

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[d/l]

        I think YAPE::Regex::Explain is an excellent tool for those learning the basics. However, it does have its LIMITATIONS:

        "There is no support for regular expression syntax added after Perl version 5.6, particularly any constructs added in 5.10. ..."

        — Ken

[reply]

        Explain is out of date. \K is no longer just K, but causes stuff to the left to not be included in $&.

[reply]

      The '\K' and '(?=...)' parts are explained in Lookaround Assertions under "perlre: Extended Patterns".

      You can use Regexp::Debugger to see what's happening (step-by-step) as a regex is processed. I find this to be a very useful tool.

      — Ken

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