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Can I do an if-then-else regex mixing match and substitution?

poly4life
Novice

Mar 25, 2011, 1:46 PM

Post #1 of 7 (4681 views)

Can I do an if-then-else regex mixing match and substitution?	Can't Post

Hello,

Is it possible to do mix match and substitution regex's inside an if-then-else regex conditional?

example:

(?(match)s/pattern/replacement/g)

It's not even pseudo-code, just a concept of what I think it might look like, but is this possible?

FYI, I'm searching..matching in a text file. If the text file matches the pattern, then do a search..replace in another area of the text file.

Thank you.

(This post was edited by poly4life on Mar 25, 2011, 1:51 PM)

FishMonger
Veteran

Mar 25, 2011, 1:56 PM

Post #2 of 7 (4675 views)

Re: [poly4life] Can I do an if-then-else regex mixing match and substitution? [In reply to]	Can't Post

The description of what you want to do is not veery clear, but maybe what you're looking for is zero width assertions.

http://perldoc.perl.org/perlreref.html#EXTENDED-CONSTRUCTS

If not, then provide more details.

miller
User

Mar 25, 2011, 2:11 PM

Post #3 of 7 (4672 views)

Re: [poly4life] Can I do an if-then-else regex mixing match and substitution? [In reply to]	Can't Post

Yes, such things are possible, but it's almost certainly better to just break up your logic instead of trying to make a complicated regex.

If you have sample data and conditions we probably could advise you better.

- Miller

poly4life
Novice

Mar 25, 2011, 2:11 PM

Post #4 of 7 (4672 views)

Re: [FishMonger] Can I do an if-then-else regex mixing match and substitution? [In reply to]	Can't Post

Yes, I believe those if-then-else conditionals. But it appears, and I could be wrong, that it applies for match regex, not any other type of regex. I want to integrate both match and substitution regex with these conditionals.

see this link: http://www.regular-expressions.info/conditional.html

Normally, I believe it's just "if" this matches, "then" check if that matches, "else" check if yet-another-thing matches.

I have a text file, right.

Code

Hello World 
 
 
Foo 
Bar

I want to do a search..match on 'Foo'. If it finds 'Foo' in this file, then do a search..replace on 'Hello World'. If it finds 'Hello World', then substitute 'Hello World' for 'Hello Earth'. You see what I mean? If 'Foo' was not present in this text file, it wouldn't even bother looking for 'Hello World'.

Could I do this in a script like Perl? Sure, I could. But I understand Regex provides some sort of condtional functionality and I thought why not just use regex if it has this feature.

Thank you.

FishMonger
Veteran

Mar 25, 2011, 2:53 PM

Post #5 of 7 (4667 views)

Re: [poly4life] Can I do an if-then-else regex mixing match and substitution? [In reply to]	Can't Post

Is this what you're after?

Code

use warnings; 
use strict; 
use File::Slurp; 
 
my $data = read_file('test.txt'); 
 
if ( $data =~ /Foo/ ) { 
    $data =~ s/Hello World/Hello Earth/; 
    print $data; 
} 
else { 
    print "Foo not found\n"; 
}

You can also do the substitution in the conditional test.

Code

if ( $data =~ /Foo/ and $data =~ s/Hello World/Hello Earth/ ) { 
    print $data; 
}

(This post was edited by FishMonger on Mar 25, 2011, 2:57 PM)

miller
User

Mar 25, 2011, 3:52 PM

Post #6 of 7 (4663 views)

Re: [poly4life] Can I do an if-then-else regex mixing match and substitution? [In reply to]	Can't Post

Follow Fishmongers advice. Doing that in a single regex is possible, but messy.

Code

use warnings;  
use strict;  
use File::Slurp;  
  
my $data = read_file('test.txt');  
  
if ( $data =~ /^(?=.*Foo)(.*?)Hello World/${1}Hello Earth/) {  
	print $data;  
} else {  
	print "Foo not found\n";  
}

poly4life
Novice

Mar 28, 2011, 8:41 AM

Post #7 of 7 (4645 views)

Re: [FishMonger] Can I do an if-then-else regex mixing match and substitution? [In reply to]	Can't Post

Thank you for the suggestions.

miller's suggestion is pretty much what I was trying to accomplish.

However, I am executing this in in-place edit mode in a batch file and it's failing on the vertical bar character in my regex:

Code

if (m~[\'|\"]\d{1,2}[\'|\"]~i)...

The error is "The system cannot find the path specified."

I realize that I don't need the vertical bar character, in this case, but if I do in the future, this could be an issue. Now, I tried modifying it a couple of different ways to narrow in on the problem.

These work:

Code

if (m~[\'\"]\d{1,2}[\'\"]~i)...

Code

if (m~[\'|\"]\d{1,2}[\'\"]~i)...

This does not:

Code

if (m~[\'\"]\d{1,2}[\'|\"]~i)...

I tried escaping the vertical bar but I got the same error:

Code

if (m~[\'\"]\d{1,2}[\'\|\"]~i)...

Does anybody know what's the problem?

Update: I did manage to find the solution. The problem is I had to precede the vertical bar with the caret (^) symbol and now it enters the if loop, due to the fact that i'm executing a Perl script inside a batch file.

I also discovered that doing a match, followed by a substitution using in-place edit mode won't replace in the file. I think this is also due to the batch process. FYI, I'm using

Code

for %%X in (*.txt) do perl -pi.bak -e "Perl code" %%X

What I'm wondering is is it possible to use look-ahead or look-behind assertions in substitutions or are assertions for matching only? Either way, it appears that in-place edit won't work here and I'll have to follow your suggestions to a T, running separate perl script.

Thank you for your help.

Thank you.

(This post was edited by poly4life on Mar 28, 2011, 9:34 AM)

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