Replace multiple lines in text file

Question

I have text files containing the text below (amongst other text)

DIFF_COEFF= 1.000e+07,1.000e+07,1.000e+07,1.000e+07,
1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,
1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,
1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,
1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,
1.000e+07,1.000e+07,1.000e+07,1.000e+07,1.000e+07,4.000e+05,

and I need to replace it with the following text:

DIFF_COEFF= 2.000e+07,2.000e+07,2.000e+07,2.000e+07,
2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,
2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,
2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,
2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,
2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,8.000e+05,

Each line above corresponds to a new line in the text file.

After some googling, I thought making use of Perl in the following might work, but it did not. I got the error message

Illegal division by zero at -e line 1, <> chunk 1

s_orig='DIFF_COEFF=*4.000e+05,'

s_new='DIFF_COEFF= 2.000e+07,2.000e+07,2.000e+07,2.000e+07,\n2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,\n2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,\n2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,\n2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,\n2.000e+07,2.000e+07,2.000e+07,2.000e+07,2.000e+07,8.000e+05,'

perl -0 -i -pe "s:\Q${s_orig}\E:${s_new}:/igs" file.txt

Does anyone here know the right way to do this?

Edit - some more details: the text after this block is "DIFF_COEFF_Q=" followed by the same set of numbers, so I need to search for and replace the specific lines shown. The text files are not very large in size.

Answer 1

Copy the file over to a new one, except that within the range of text between these markers drop the replacement text instead. Then move that file to replace the original, as it may be needed judging by the attempted perl -0 -i in the question.

Note that when changing a file we have to build new content and then replace the file. There are a few ways to do this and modules that make it easier, shown further below.

The code below uses the range operator and the fact that it returns the counter for lines within the range, 1 for the first and the number ending with E0 for the last. So we don't copy lines inside that region while we write the replacement text (and the post-region-end marker) on the last line.

I consider the region of interest to end right before DIFF_COEFF_Q= line, per the question edit.

use warnings;
use strict;
use feature 'say';
use File::Copy 'move';

my $replacement = "replacement text";

my $file     = 'input.txt';
my $out_file = 'new_' . $file;

open my $fh_out, '>', $out_file or die "Can't open $out_file: $!";
open my $fh,     '<', $file     or die "Can't open $file: $!";

while (<$fh>) 
{
    if (my $range_cnt = /^\s*DIFF_COEFF\s*=/ .. /^\s*DIFF_COEFF_Q\s*=/) #/
    {
        if ($range_cnt =~ /E0$/)
        {
            print $fh_out $replacement;  # may need a newline
            print $fh_out $_;         
        }
    }   
    else { 
        print $fh_out $_; 
    }
}
close $fh     or die "Can't close $file: $!";      # don't overwrite original
close $fh_out or die "Can't close $out_file: $!";  # if there are problems

#move $out_file, $file or die "Can't move $file to $out_file: $!";

Uncomment the move line once this has been tested well enough on your actual files, if you want to replace the original. You may or may not need a newline after $replacement, depending on it.

An alternative is to use flags for entering/leaving that range. But this won't be cleaner since there are two distinct actions, to stop copying when entering the range and write replacement when leaving. Thus multiple flags need be set and checked, what may end up messier.

If the files can't ever be huge it is simpler to read and process the file in memory. Then open the same file for writing and dump the new content

my $text = do {  # slurp file into a scalar
    local $/; 
    open my $fh, '<', $file or die "Can't open $file: $!"; 
    <$fh> 
};

$text =~ s/^\s*DIFF_COEFF\s*=.*?(\n\s*DIFF_COEFF_Q)/$replacement$1/ms;

# Change $out_file to $file to overwrite
open my $fh_out, '>', $out_file or die "Can't open $out_file: $!";
print $fh_out $text;

Here /m modifier is for multiline mode in which we can use ^ for the beginning of a line (not the whole string), what is helpful here. The /s makes . match a newline, too. Also note that we can slurp a file with Path::Tiny as simply as:  my $text = path($file)->slurp;

Another option is to use Path::Tiny, which in newer versions has edit and edit_lines methods

use Path::Tiny;
                      # NOTE: edits $file in place (changes it)
path($file)->edit( 
    sub { s/DIFF_COEFF=.*?(\n\s*DIFF_COEFF_Q)/$replacement$1/s } 
);

For more on this see, for example, this post and this post and this post.

The first and last way change the inode number of the file. See this post if that is a problem.

Answer 2

It's an interesting error that you've made and I can see what has led you to make it. But I don't think I've ever seen anyone else make the same mistake :-)

Your substitution statement is this:

s:\Q${s_orig}\E:${s_new}:/igs

So you've decided to use : as the delimiter of the substitution operator. But you want to use the options i, g and s and everywhere you've seen people talk about options on a substitution operator, they talk about using / to introduce the options. So you've added /igs to your substitution operator.

But what you've missed (and I completely understand why) is that the / that comes before the options is actually the closing delimiter of the standard, s/.../.../, version of the substitution operator. If you change the delimiter (as you have done) then your altered closing delimiter is all you need.

In your case, Perl doesn't expect the / as it has already seen the closing delimiter. It, therefore, decides that the / is a division operator and tries to divide the result of your substitution by igs. It interprets igs as zero and you get your error.

The fix is to remove that / so:

s:\Q${s_orig}\E:${s_new}:/igs

becomes:

s:\Q${s_orig}\E:${s_new}:igs