G'day Zu,

Welcome to the monastery.

I get the feeling that there's some aspect of this that you haven't told us about. For both strings, you have three capture groups but discard $2 in each case. Also, your replacements both have "\nfield2: valueB\n" hard-coded.

However, you've said "The regex below works"; on that basis, this solution is not "slow" for either string. 

#!/usr/bin/env perl -l

use strict;
use warnings;

use Time::HiRes qw{time};

my $no_field2 = "field1: valueA\nsome\nlines\nhere\nfield3: valueC\n"
                . "........................................\n" x 1000;
my $has_field2 = "field1: valueA\nfield2: valueB\nfield3: valueC\n"
                . "........................................\n" x 1000;
my $middle = "\nfield2: valueB\n";

my $re = qr{(^field1:.*?$).*?(^field3:)}ms;

replace_middle($_, $middle, $re) for ($no_field2, $has_field2);

sub replace_middle {
    my ($string, $middle, $re) = @_;

    print '-' x 40;
    print "Start:\n", substr $string, 0, 60;

    my $t0 = time;

    $string =~ s/$re/$1$middle$2/;

    my $t1 = time;

    print "Finish:\n", substr $string, 0, 60;
    print 'Time:   ', $t1 - $t0;
}

[download]

Output:

----------------------------------------
Start:
field1: valueA
some
lines
here
field3: valueC
..............
Finish:
field1: valueA
field2: valueB
field3: valueC
...............
Time:   6.89029693603516e-05
----------------------------------------
Start:
field1: valueA
field2: valueB
field3: valueC
...............
Finish:
field1: valueA
field2: valueB
field3: valueC
...............
Time:   1.62124633789062e-05

[download]

-- Ken

In reply to Re: Efficient method to replace middle lines only when no match by kcott
in thread Efficient method to replace middle lines only when no match by Zu

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.