in reply to sequential substitutions

As stated it is simple enough, but there is usually more to the story, requiring more complex parsing than just detecting digits and substituting. 
( echo '<foo>3</foo>'; echo '<foo>14</foo>'; echo '<foo>159</foo>') | 
+perl -e '

my $x = 0;
while (my $l = <>) {
    $x++; $l =~ s/\d+/$x/;
    print $l;
}'
<foo>1</foo>
<foo>2</foo>
<foo>3</foo>

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Re^2: sequential substitutions
by Anonymous Monk on Aug 03, 2018 at 10:48 UTC

    There is a little bit more to it (but not much), and I do apologize for over-simplifying.

    Let's suppose the code is actually:

    <foo>3</foo>
<bar>a</bar>
<foo>14</foo>
<bar>bc</bar>
<foo>159</foo>
<bar>def</bar>
[...]

    [download]

    I want to simply echo lines that do not contain the "foo" tag to the output file.

[reply]
[d/l]

      Like this? 

      use strict;
use warnings;
my $i = 0;
while(<DATA>){
    s/<foo>.*?<\/foo>/"<foo>".++$i."<\/foo>"/e;
    print;
}
__DATA__
<foo>3</foo>
<bar>a</bar>
<foo>14</foo>
<bar>bc</bar>
<foo>159</foo>
<bar>def</bar>

      [download]
[reply]
[d/l]

        Basically, yes, though I am going to be doing it all at once (using a technique described in a later answer) - reading in the entire file, and then processing it with:

        $fragment =~ s/<foo>\d+<\/foo>/'<foo>' . $i++ . '<\/foo>'/eg;

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        Thank you.

[reply]
[d/l]

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