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matching lines in a long string

by Cristoforo (Curate)
on May 30, 2020 at 00:29 UTC ( #11117487=perlquestion: print w/replies, xml ) Need Help??

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

Hello monks!

Been working on this problem and can't find my error. I'm trying to use the m modifier to read a large text in line by line. I want to use the inline form, (?m) rather than following the regex slash with m.

#!/usr/bin/perl use strict; use warnings; use feature 'say'; my $s = <<'EOF'; int t; //variable t t->a=0; //t->a does;; something printf("\nEnter the Employee ID : "); scanf("%d", ptrx->eid); //employee id ptrx->eid printf("\nEnter the Employee Name : "); scanf("%s", ptr->name); return 0; EOF for my $line (split/(?m)\n(?=$)/, $s) { say '<' . $line . '>'; }
I wonder if anyone can see why it isn't working. The output is:

C:\Old_Data\perlp>perl test3.pl <int t; //variable t t->a=0; //t->a does;; something printf("\nEnter the Employee ID : "); scanf("%d", ptrx->eid); //employee id ptrx->eid printf("\nEnter the Employee Name : "); scanf("%s", ptr->name); return 0;>
It isn't reading 'line by line'. Thanks for any help you might offer!

Update: It works ok without the extra specifiers if i just do for my $line (split/\n/, $s)

But wondering why.

Replies are listed 'Best First'.
Re: matching lines in a long string
by jwkrahn (Monsignor) on May 30, 2020 at 03:09 UTC

    It also works like this:

    for my $line ( split /^/, $s )
Re: matching lines in a long string
by haukex (Bishop) on May 30, 2020 at 19:59 UTC

    Athanasius and AnomalousMonk already explained the issue. I just wanted to add: The most performant way is the following; it avoids splitting the lines into a list first.

    use warnings; use strict; my $s = <<'EOF'; int t; //variable t t->a=0; //t->a does;; something printf("\nEnter the Employee ID : "); scanf("%d", ptrx->eid); //employee id ptrx->eid printf("\nEnter the Employee Name : "); scanf("%s", ptr->name); return 0; EOF open my $fh, '<', \$s or die $!; while ( my $line = <$fh> ) { chomp($line); print "<$line>\n"; } close $fh;
Re: matching lines in a long string
by Athanasius (Bishop) on May 30, 2020 at 04:21 UTC

    Hello Cristoforo,

    Within the look-ahead assertion (?=$), the metacharacter $ means:

    Match the end of the string (or before newline at the end of the string; or before any newline if /m is used) (from “Metacharacters” in perlre#The-Basics)

    Since it comes after the newline within the regex pattern — and your string contains no consecutive newlines — it can match only at the very end of the string. This explains the behaviour you’re seeing.

    If you remove the look-ahead:

    for my $line (split/(?m)\n/, $s) {

    the output is displayed line-by-line, as expected.

    Hope that helps,

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

      I disagree slightly with your narrative. A split pattern of  /(?m)\n(?=$)/ will split on any newline that is followed immediately by a newline (i.e., consequtive newlines) or on a newline at the end of the string (producing an empty string). This can be seen to be happening with modified text (and with a split LIMIT of -1 to preserve trailing null fields):

      c:\@Work\Perl\monks>perl use strict; use warnings; use feature 'say'; my $s = <<'EOF'; int t; //variable t t->a=0; //t->a does;; something printf("\nEnter the Employee Name : "); scanf("%s", ptr->name); return 0; EOF for my $line (split/(?m)\n(?=$)/, $s, -1) { say '<' . $line . '>', "\n"; } __END__ <int t; //variable t t->a=0; //t->a does;; something> < printf("\nEnter the Employee Name : "); scanf("%s", ptr->name); return 0;> <>

      I agree that the simplest and best approach is to split on what the OPer wants to split on, i.e., newlines, and forget about the  /m modifier and everything else.


      Give a man a fish:  <%-{-{-{-<

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