I have a file whose contents have been slurped into a string. I'm trying to find an efficient solution that will replace content between two matching lines with different content, but only if there isn't a matching line in between.

So if the relevant portion of a proper file looked like this:

field1: valueA
field2: valueB
field3: valueC

[download]

then I don't want to make changes. But if it looked like this:

field1: valueA
some
lines
here
field3: valueC

[download]

then I want to correct it to look like the output of the first example.

The field names are fixed but the values could be anything. The regex below works, but when field2 exists it's quite slow. Likely due to the combination of greedy .* and negative look-ahead.

# this is quick
my $no_field2 = "field1: valueA\nsome\nlines\nhere\nfield3: valueC\n";
$no_field2 .= "........................................\n" x 1000;

$no_field2 =~ s/(field1:.*?$)\n(?!^field2:)(.*$)\n(^field3:)
               /$1\nfield2: valueB\n$3
               /msx;

# this is slow
my $has_field2 = "field1: valueA\nfield2: valueB\nfield3: valueC\n";
$has_field2 .= "........................................\n" x 1000;

$has_field2 =~ s/(field1:.*?$)\n(?!^field2:)(.*$)\n(^field3:)
                /$1\nfield2: valueB\n$3
                /msx;

[download]

What is the most efficient, least code solution?

In reply to Efficient method to replace middle lines only when no match by Zu

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