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more regex

nipper

Trying to replace a string in a bunch of DCRs, with the exception of one part.

First thought:

$chamge_from="something";
$change_to="else";

$dont_change="something I want to preserve";

slurp DCR into $txt

How can I change $txt any instance of $change_from that is not in $dont_change ?

my current answer is

$txt =~ s/$dont_change/"ANDYSAYSSAVETHIS"/g;
$txt =~ s/$change_from/$change_to/g;
$txt =~ s/"ANDYSAYSSAVETHIS"/$dontchange/g;

Got to think there is a better way.

Andy

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Comments

Adam Stoller

Actually, that's probably not a bad way to do it - simple and accurate - though you want to use the 's' modifier in addition to the 'g' modifier on the substitutions since you may have embedded newlines with $txt;

--fish
Senior Consultant, Quotient Inc.
http://www.quotient-inc.com

Migrateduser

The "right" way to do it would be with lookahead and lookbehind assertions (if my assumptions about what he means about the text he doesn't want to change being 'inside' something he does want to change are correct) - but, I like the approach he's using. It seems robust, and it would be easy for even a complete newcomer to understand what he's doing.

If you can give us a more accurate sample of the data, I might be able to tell you what the regex would look like with some assertions.

Migrateduser

That would certainly work but what about:

s/$change_from([^$dont_change])/$change_to/gs;

For your specific example, it would work but if there are variations of $dont_change, Perl may complain.

Dave

dave@simpleinternet.com
+USA (215) 962-5153

Migrateduser

s/$change_from([^$dont_change])/$change_to/gs;

This syntax doesn't work with strings, only sets of characters. This won't do what you want it to do.

Dave Smith
Sr. Software Engineer
Nike, Inc.
(503) 671-4238
DavidH.Smith@nike.com

BSS-2460_02.JPG

Migrateduser

That wouldn't work (again, assuming my assumptions about what his data is actually like are true).

in your example, whatever letters were in $dont_change would become a character class. Let's say $dont_change eq "cat".

Then what you have reads "match the contents of $change_from, followed by one character that is not a c, an a, or a t) and replace that with the string in $change_to.

Maybe you were going after something like:

s/$change_from(?!dont_change)/$change_to/gs;

but that's back to my idea of using the lookahead assertions. I think for his example though, since he says "in", that you also would need to do some look behind as well.

jbonifaci

Andy,

You can just count on the regex being greedy and do something like the code below. Being greedy, it will always match $doNotChange if it can, otherwise it will match $changeFrom. You then test to see if it matched just $changeFrom, and if it did, replace it, otherwise leave it alone.

use strict;

my $text = <<TEXT;
This is text, feel free to change it.
This is my text, leave it alone.
This is text, feel free to change it.
This is my text, leave it alone.
This is text, feel free to change it.
This is my text, leave it alone.
TEXT

my $changeFrom = "text";
my $changeTo = "your text";
my $doNotChange = "my text";

$text =~ s/($changeFrom|$doNotChange)/eval($1eq"$changeFrom")?$changeTo:$1/eg;

print $text;

Hope this helps,
~Jeff