Substitute Text with RegEX in external file Perl - regex

Question

I am trying to have a script that will update a variable in an input file. The RegEx matches but it does not perform the substitution. What am I doing wrong.
sub updateInputDeck {
my $powerLevel = shift;
my $file = $outputFiles{input};
open INPUTFILE,"<",$file or die "Cannot open file $file $!\n";
while (<INPUTFILE>) {
if (s/((?<=\s{3}RP\s{2}=\s{2})\d+)/$powerLevel/) {
print $_;
print "Updating Input File for Power Level: $powerLevel";
}
}
close INPUTFILE;
}
UPDATE
I am trying to update the file pointed to in the filehandle. Can I only do this via a print statement. If that is the case I just want to reprint that one line. Is that possible?

Answer 1

You can use perl's in-place editing:
sub updateInputDeck {
my $powerLevel = shift;
my $file = $outputFiles{input};
local #ARGV = ($file);
local $^I = '.bac';
while( <> ){
s/((?<=\s{3}RP\s{2}=\s{2})\d+)/$powerLevel/;
print;
}
#unlink "$file$^I" or die "Can't delete backup";
return;
}
Also note, that your use of a global $outputFiles{input} as a parameter to your function is a bad style practice. Instead pass it as a parameter to your function as well.

Answer 2

I think you have to do it in 2 passes. First read the whole file into an array, edit it locally and write the whole thing back out, overwriting the original file.
open INPUTFILE,"<$file" or die "Cannot open file $file $!\n";
my #lines = <INPUTFILE>; # Read in entire file.
close INPUTFILE;
open INPUTFILE,">$file" or die "Cannot open file $file $!\n";
foreach $line (#lines) {
$line =~ s/((?<=\s{3}RP\s{2}=\s{2})\d+)/$powerLevel/;
print INPUTFILE $line;
}
close INPUTFILE;