Re: Efficient method to replace middle lines only when no match

A line-by-line approach might actually be more efficient/maintainable, but if the file's already slurped, maybe something like this (needs Perl 5.10+ for \K but this could be worked around):

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 my $s = qq{yada\n}
       . qq{field1: valueA\n}
       . qq{some\n}
       . qq{lines\n}
       . qq{here\n}
       . qq{field3: valueC\n}
       . qq{blah blah\n}
       ;
 print qq{[[$s]]};
 ;;
 my $replace = qq{field2: valueB\n};
 ;;
 $s =~ s{
     ^   field1: \s valueA \n \K
     (?! \Q$replace\E)  .*?
     (?= field3: \s valueC \n)
   }
   {$replace}xmsg;
 print qq{[[$s]]};
"
[[yada
field1: valueA
some
lines
here
field3: valueC
blah blah
]]
[[yada
field1: valueA
field2: valueB
field3: valueC
blah blah
]]
[download]

Update 1: Changed (?! $replace) to (?! \Q$replace\E)

Update 2: I don't know if I just missed it or it was added after I began my reply, but the code at the bottom of the OP looks a lot like my post, and in fact is better since it takes care of variable value fields as mine does not. ?!?

Comment on Re: Efficient method to replace middle lines only when no match Select or Download Code

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Re^2: Efficient method to replace middle lines only when no match by Zu (Initiate) on Mar 20, 2014 at 06:54 UTC
Thanks for using \K in your post, it made me think a bit more and I came up with a solution that doesn't have a penalty when there's a negative match: `# slow no longer my $has_field2 = "field1: valueA\nfield2: valueB\nfield3: valueC\n"; $has_field2 .= "........................................\n" x 1000; $has_field2 =~ s/field1:[\n]\n\K(?!field2:).(?=\nfield3:) /field2: valueB /msx;` [download] Also, I didn't edit my original post.	[reply] [d/l]
Re^3: Efficient method to replace middle lines only when no match by AnomalousMonk (Archbishop) on Mar 20, 2014 at 15:49 UTC
`$has_field2 =~ s/field1:[\n]\n\K(?!field2:).(?=\nfield3:) /field2: valueB /msx;` [download] Shouldn't the `[\n]\n` after `field1:` in the search pattern be `[^\n]\n` (negated class) instead? The `.` in the search pattern is greedy. Your test string has only one occurrence of the `"field1: valueA\nfield2: valueB\nfield3: valueC\n"` substring. What happens if you test against many occurrences (see below — BTW, I have an updated version of this script that, among other things, uses hi-res timing if you're interested)? Won't the greedy `.` just gobble all intervening occurrences? You have a newline and a lot of blank space in your replacement string; is this what you want? `c:\@Work\Perl\monks\Zu>perl -wMstrict -le "my $s = 'xxxyxxx'; $s =~ s/y /FOO /xms; print qq{'$s'}; " 'xxxFOO xxx'` [download]	[reply] [d/l] [select]