Replace text between START & END strings excluding the END string in perl

Question

I was going through examples and questions on the web related to finding and replacing a text between two strings (say START and END) using perl. And I was successful in doing that with the provided solutions. In this case my START and END are also inclusive to replacement text. The syntax I used was s/START(.*?)END/replace_text/s to replace multiple lines between START and END but stop replacing when first occurrence of END is hit.

But I would like to know how to replace a text between START and END excluding END like this in perl only.

Before:

START
I am learning patten matching.
I am using perl.
END

After:

Successfully replaced.
END

Robin · Accepted Answer · 2014-05-22 10:39:39Z

6

To perform the check but avoid matching the characters you can use positive look ahead:

s/START.*?(?=END)/replace_text/s

answered May 22, 2014 at 10:39

Robin

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AntonH · Accepted Answer · 2014-05-22 10:41:29Z

3

One solution is to capture the END and use it in the replacement text.

s/START(.*?)(END)/replace_text$2/s

answered May 22, 2014 at 10:41

AntonH

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Lee Duhem · Accepted Answer · 2014-05-22 12:34:54Z

2

Another option is using range operator .. to ignore every line of input until you find the end marker of a block, then output the replace string and end marker:

#!/usr/bin/perl

use strict;
use warnings;

my $rep_str = 'Successfully replaced.';

while (<>) {
    my $switch = m/^START/ .. /^END/;
    print unless $switch;
    print "$rep_str\n$_" if $switch =~ m/E0$/;
}

It is quite easy to adapt it to work for an array of string:

foreach (@strings) {
     my $switch = ...
     ...
}

edited May 22, 2014 at 12:34

answered May 22, 2014 at 10:48

Lee Duhem

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1 Comment

Lee Duhem Over a year ago

@IlmariKaronen Another print is needed, because there may be other lines outside those START .. END block. That feature is part of my Perl template, I forget to remove it when I paste that code. Thank you for pointed out the flaws in my answer.

JRFerguson · Accepted Answer · 2014-05-22 11:14:15Z

1

To use look-around assertions you need to redefine the input record separator ($/) (see perlvar), perhaps to slurp the while file into memory. To avoid this, the range ("flip-flop") operator is quite useful:

while (<>) {
    if (/^START/../^END/) {
        next unless m{^END};
        print "substituted_text\n";
        print;
    }
    else {
        print;
    }
}

The above preserves any lines in the output that precede or follow the START/END block.

edited May 22, 2014 at 11:14

answered May 22, 2014 at 11:07

JRFerguson

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