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match, but do not match

JasperD
Novice

Dec 16, 2010, 4:59 AM

Post #1 of 6 (2175 views)

match, but do not match	Can't Post

Hi everyone,

Trying to substitute some matched strings does not seem to work out for me.

Consider the following text: ABC"DEF"GHI"JKL"

I remove the "DEF" and "JKL" as follows:

s/".*?"//g or s/"[^"]*"//g nicely gives: ABCGHI

However, my problem is more complicated, it has a @ to tell the " should not match:

eg: ABC@"DEF@"GHI"JKL"

The result I am looking for is: ABC@"DEF@"GHI (or better if possible ABC"DEF"GHI)

I have tried stuff like:

s/[^@]".*?"//g (ungreedy) or s/([^@]"|")[^"]*"//g or s/([^@]"&")[^"]*"//g

One of the main problems with the [^@] notation in combination with s///g is that the matching character (not @) will be substituted with nothing as well.

How do I tell the system to match " but not match @" (and possibly to remove the @) ?

Thanks

shawnhcorey
Enthusiast

Dec 16, 2010, 5:59 AM

Post #2 of 6 (2170 views)

Re: [JasperD] match, but do not match [In reply to]	Can't Post

You need to use one of the lookbehind patterns. This may not work if there's a double quote at the start of a line.

Code

#!/usr/bin/perl 
 
use strict; 
use warnings; 
 
while( <DATA> ){ 
  chomp; 
  print "$_  ->  "; 
  s{ (?<!\@) \" [^"]* \" }{}gmsx; 
  print "$_\n"; 
} 
 
__DATA__ 
ABC"DEF"GHI"JKL" 
ABC@"DEF@"GHI"JKL"

See `perldoc perlre`.

__END__

I love Perl; it's the only language where you can bless your thingy.

Perl documentation is available at perldoc.perl.org. The list of standard modules and pragmatics is available in perlmodlib.

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JasperD
Novice

Dec 17, 2010, 2:16 AM

Post #3 of 6 (2097 views)

Re: [shawnhcorey] match, but do not match [In reply to]	Can't Post

Thanks for the answer already,

The look-behind pattern is probably going to help me and the code you gave seems to work for me.
However, your post leads to ask me a couple of more questions since I would like to know what I am doing.
I am not familiar with this syntax:

Code

 s{ (?<!\@) \" [^"]* \" }{}gmsx;

Where can I find info about "gmsx"? A search on preldoc does not seem to give any results,
What is the {}?
The syntax you use, can this be optimized (disable backtracking, disable buffering)?

Like the double quotes string match example from perlre:

Code

 /"([^"\\]+|\\.)*"/

Which according to perldoc can be most efficiently performed when written as:

Code

 /"(?:[^"\\]++|\\.)*+"/

So, can my original question also be written as a optimized single regex pattern to be used in s///g?

shawnhcorey
Enthusiast

Dec 17, 2010, 6:32 AM

Post #4 of 6 (2083 views)

Re: [JasperD] match, but do not match [In reply to]	Can't Post

In Reply To

Where can I find info about "gmsx"? A search on preldoc does not seem to give any results,

See `perldoc perlre` and search for /Character Classes and other Special Escapes/

In Reply To

What is the {}?

They are delimiters for the pattern and replacement is s

In Reply To

The syntax you use, can this be optimized (disable backtracking, disable buffering)?

Shawn's second Rule of Programming: First you make it work, and only then you make it better.

__END__

I love Perl; it's the only language where you can bless your thingy.

Perl documentation is available at perldoc.perl.org. The list of standard modules and pragmatics is available in perlmodlib.

Get Markup Help. Please note the markup tag of "code".

JasperD
Novice

Dec 17, 2010, 7:28 AM

Post #5 of 6 (2078 views)

Re: [shawnhcorey] match, but do not match [In reply to]	Can't Post

Your replies sincerely appreciated,

In Reply To

Shawn's second In Reply ToRule of Programming: First you make it work, and only then you make it better.

Disagreeing, I am not here to make it work, but to learn. (Besides, it already works).

My last question remains unanswered.
I would like to know how to write this as a single statement and how to optimize it.

Does anyone else have thoughts on this?

JasperD
Novice

Dec 19, 2010, 4:38 AM

Post #6 of 6 (2018 views)

Re: [JasperD] match, but do not match [In reply to]	Can't Post

After a bit more puzzling I have decided to use the following:

Code

#!/usr/bin/perl 
use strict; 
use warnings; 
while( <DATA> ) { 
   chomp ; 
   print "$_  ->  " ; 
   s/(?:(?<!\@)"[^"]*+"|@(?="))//g ; 
   print "$_\n"; 
} 
__DATA__ 
"ABC"DEF"GHI"JKL"MNO 
ABC"DEF"GHI"JKL" 
ABC@"DEF@"GHI"JKL" 
A@BC@@"D@EF@"G@HI"JKL"

Which also removes @":

Code

"ABC"DEF"GHI"JKL"MNO   ->  DEFJKL"MNO 
ABC"DEF"GHI"JKL"       ->  ABCGHI 
ABC@"DEF@"GHI"JKL"     ->  ABC"DEF"GHI 
A@BC@@"D@EF@"G@HI"JKL" ->  A@BC@"D@EF"G@HI

Thanks for pointing out to me to use Look-Around Assertions (which I now also applied to remove the @ sign).

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