How to replace a string in an existing file in Perl?

Question

I want to replace word \"blue\" to \"red\" in all text files named as 1_classification.dat, 2_classification.dat and so on. I want to edit the same file so I tried this code

Answer 1

None of the existing answers here has provided a complete example of how to do this from within a script (not a one-liner). Here is what I did:

rename($file, $file.'.bak');
open(IN, '<'.$file.'.bak') or die $!;
open(OUT, '>'.$file) or die $!;
while(<IN>)
{
    $_ =~ s/blue/red/g;
    print OUT $_;
}
close(IN);
close(OUT);

Answer 2

It can be done using a single line:

perl -pi.back -e 's/oldString/newString/g;' inputFileName

Pay attention that oldString is processed as a Regular Expression.
In case the string contains any of {}[]()^$.|*+? (The special characters for Regular Expression syntax) make sure to escape them unless you want it to be processed as a regular expression.
Escaping it is done by \, so \[.

Answer 3

$_='~s/blue/red/g';

Uh, what??

Just

s/blue/red/g;

or, if you insist on using a variable (which is not necessary when using $_, but I just want to show the right syntax):

$_ =~ s/blue/red/g;

Answer 4

Anything wrong with a one-liner?

$ perl -pi.bak -e 's/blue/red/g' *_classification.dat

---

Explanation

• -p processes, then prints <> line by line
• -i activates in-place editing. Files are backed up using the .bak extension
• The regex substitution acts on the implicit variable, which are the contents of the file, line-by-line