Replace string in shell using sed or Perl? - perl

Question

How can I replace the following string:
<value>-myValue</value>
<value>1234</value>
And make it to be:
<value>-myValue</value>
<value>0</value>
Please take into account that there is a line break.

Answer 1

Script
sed -e '/<value>-myValue</,/<value>/{ /<value>[0-9][0-9]*</ s/[0-9][0-9]*/0/; }' data
From a line containing <value>-myValue< to the next line containing <value>, if the line matches <value>XX< where XX is a string of one or more digits, replace the string of digits with 0.
Input
This is not something to change
<value>-myValue</value>
<value>1234</value>
<value>myValue</value>
<value>1234</value>
nonsense
<value>-myValue</value>
<value>abcd</value>
<value>-myValue</value>
<value>4321</value>
stuffing
Output
This is not something to change
<value>-myValue</value>
<value>0</value>
<value>myValue</value>
<value>1234</value>
nonsense
<value>-myValue</value>
<value>abcd</value>
<value>-myValue</value>
<value>0</value>
stuffing

Answer 2

If this is XML, TLP is right that an XML parser would be superior. Continuing on with your sed approach, however, consider:
$ sed '/<value>-myValue/ {N; s/<value>[[:digit:]]\+/<value>0/}' file
<value>-myValue</value>
<value>0</value>

Answer 3

You can possibly simplify this a bit, depending on what criteria you specifically want to use:
#!/usr/bin/env perl
use strict;
use warnings;
use XML::Twig;
my $twig = XML::Twig->new( 'pretty_print' => 'indented_a' )->parse( \*DATA );
foreach my $value ( $twig->findnodes('//value') ) {
if ( $value->trimmed_text eq '-myValue'
and $value->next_sibling('value')
and $value->next_sibling('value')->text =~ m/^\d+$/ )
{
$value->next_sibling('value')->set_text('1234');
}
}
$twig->print;
__DATA__
<root>
<value>-myValue</value>
<value>0</value>
</root>
This outputs:
<root>
<value>-myValue</value>
<value>1234</value>
</root>
It parses your XML.
Looks for all nodes with a tag of value.
Checks that it has a sibling.
Checks that sibling is 'just numeric' e.g. matching regex ^\d+$
replaces the content of that sibling with 1234.
And will work on XML regardless of formatting, which is the problem with XML - pretty fundamentally there's a bunch of entirely valid things you can do that are semantically identical in XML.