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Strange behavior of variable in substitution expression

Fred
User

Aug 20, 2012, 3:34 AM

Post #1 of 9 (7384 views)

Strange behavior of variable in substitution expression	Can't Post

Hello,

I am working on a simple search/replace program to replace some characters by another ones in a file.

The program ask the user the substitution he wants to make.

I would like to replace a newline by a tabulation with this program but I never succeeded Crazy

If the user enters with the keyboard <\n> as a search character and then <\t> as a replacement character, the newline \n is well recognized by the program, but the tabulation \t is not. As a result, the program replace each newline by the character <\t>.

If in my program, I set :
$with = "\t" ;
Then the program works perfectly, but I did not found how to enter \t with the keyboard (I tried "\t", '\t', \t ...), nothing work...

Many thanks in advance for your help !

Code

 
# Simple search / replace program 
 
print "Replace: " ; 
my $replace = (<>) ; 
chomp $replace ; 
$replace = qr/$replace/ ; # use $replace as a regexp 
 
 
print "with: " ; 
my $with =  (<>); 
chomp $with ; 
 
 
open(FILEIN, $file_in) ; 
open(FILEOUT, "> $file_out") ; 
 
 while (<FILEIN>) { 
	s/$replace/$with/g ; 
	print FILEOUT "$_" ; 
	}

BillKSmith
Veteran

Aug 20, 2012, 4:57 AM

Post #2 of 9 (7379 views)

Re: [Fred] Strange behavior of variable in substitution expression [In reply to]	Can't Post

On windows, I can just type the Tab and Enter keys. If you want to use the escape sequence, you could test for it and convert it in your program.
Good Luck,
Bill

Fred
User

Aug 20, 2012, 5:12 AM

Post #3 of 9 (7377 views)

Re: [BillKSmith] Strange behavior of variable in substitution expression [In reply to]	Can't Post

Thank you Bill !

But now, imagine that I want to replace any space by a newline... I have to find another solution Tongue

Laurent_R
Veteran / Moderator

Aug 20, 2012, 5:32 AM

Post #4 of 9 (7375 views)

Re: [Fred] Strange behavior of variable in substitution expression [In reply to]	Can't Post

Hi,

this is not the most elegant piece of code, and there is probably a better solution, but the following code works:

Change the relevant part of your code this way:

Code

print "with: " ;  
my $with =  (<>);  
chomp $with ;  
$with = "\t" if $with eq '"\t"';

Then, when prompted to give the replacement string, type this: "\t" (actually typinq the double quotes).

I assume this will also work if you want a "\n" as the replacement string.

Fred
User

Aug 20, 2012, 7:02 AM

Post #5 of 9 (7371 views)

Re: [Laurent_R] Strange behavior of variable in substitution expression [In reply to]	Can't Post

Thank you Laurent !

I also found a similar solution ; But my solution also have some big limitations. Imagine for exemple that you want to replace a space by "\n"."a_chain_of_characters". "\n"
For a simple one character replacement it is ok but for a complex one it does not work.

Code

 
if ($with =~ m/\\([A-Z,a-z])/){ 
  my $char = $1 ; 
  switch ($char) { 
    case "t" {$with = "\t" ;} 
      case "r" {$with = "\r" ;} 
	case "n" {$with = "\n" ;} 
  } 
}

BillKSmith
Veteran

Aug 20, 2012, 10:03 AM

Post #6 of 9 (7363 views)

Re: [Fred] Strange behavior of variable in substitution expression [In reply to]	Can't Post

The ternary operator even works in much older versions of perl.

Code

my $raw_with = <>; 
chomp $raw_with; 
my $with = $raw_with eq '\r'  ? "\r" 
         : $raw_with eq '\n'  ? "\n" 
         : $raw_with eq '\t'  ? "\t" 
         : $raw_with eq '\s'  ? " " 
         :                     $raw_with 
         ; 
$string =~ s/$replace/$with/e;

Good Luck,
Bill

Fred
User

Aug 20, 2012, 10:22 AM

Post #7 of 9 (7357 views)

Re: [BillKSmith] Strange behavior of variable in substitution expression [In reply to]	Can't Post

Thank you Bill !

But why do you use the <e> option for the s/// operator ? I used <g> instead because I want to enable multiple replacements

Laurent_R
Veteran / Moderator

Aug 20, 2012, 10:32 AM

Post #8 of 9 (7355 views)

Re: [Fred] Strange behavior of variable in substitution expression [In reply to]	Can't Post

The e flag enables evaluating the replacement string as a Perl expression. You can use both g and e in the same replacement: $foo =~ s/source/target/eg;

An alternative to the ternary operator or the successive else or case statements would be to use a hash to find the replacement string from the user input.

(This post was edited by Laurent_R on Aug 20, 2012, 10:34 AM)

BillKSmith
Veteran

Aug 20, 2012, 11:40 AM

Post #9 of 9 (7349 views)

Re: [Fred] Strange behavior of variable in substitution expression [In reply to]	Can't Post

Sorry, that's a mistake, a leftover from a failed effort. Use /g
Good Luck,
Bill

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