Substitution Operator

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi i am a beginner programmer and i was foolin around with
this program:

#!/usr/bin/perl
 
   
print "What file do you want to \"detabify?\" ";
$file=<>;
chomp $file;
$/=undef;                              
open FILE, "<$file";  
$text=<FILE>;                      
close FILE;
$text=~s/\t/   /g;                    
open FILE, ">$file";
print $text;                           
close FILE;
[download]

and i was woundering how you would goet it to read from a
text file and change all the letter J's to P's.
i tryed using this: $text=~s/j/p/;
but it would only change the first letter J...
Could someone please help me with this....

Comment on Substitution Operator Download Code

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Re: Substitution Operator by blakem (Monsignor) on Aug 28, 2001 at 11:57 UTC
If you are just wanting to substitute a single letter, the above `tr` solution is recommended. However, if you wanted to replace an entire string throughout your $text variable, you'd want to use the s///g construct. Since this also works for single letters, a slightly less optimized solution than `tr/j/p/` is: `$text =~ s/j/p/g;` [download] Your code could also benefit from `use strict;` which will help you practice good coding style, and taint checking which will help you realize the inherant insecurities of passing user input directly to system calls. -Blake	[reply] [d/l] [select]
Re: Substitution Operator by bwana147 (Pilgrim) on Aug 28, 2001 at 11:52 UTC
`perl -pi.bak -e 'tr/j/p/' myfile` Read perlrun to learn everything there is to know about perl's command-line arguments, and perlop for the details about the transliteration operator `tr` --bwana147	[reply] [d/l] [select]
Re: Substitution Operator by Chady (Priest) on Aug 28, 2001 at 12:29 UTC
...moreover, you are not reading the whole file, you are just reading the first line.. the <FILEHANDLE> operator returns a single line if it is expected to return a scalar, and a list in list context.. so to read all your file, you must read it into an array: `@lines = <FILE>;` then iterate over that array.. The array is split with the `$/` variable. which is \n by default.. you can undef this variable to read the whole file as a single string. `{ local $/; $text = <FILE>; }` [download] this will read all the text into a single variable... you should look into what solution suits you better. He who asks will be a fool for five minutes, but he who doesn't ask will remain a fool for life. Chady \| http://chady.net/	[reply] [d/l] [select]
Re: Substitution Operator by Zecho (Hermit) on Aug 28, 2001 at 12:50 UTC
perlman:perlop is a great place to start, it describes the s/// function in detail.. If you really want to learn the ins and outs I recommend bookmarking it and studying it. `update: changed to[id:] link for cookie's sake`	[reply] [d/l]