CODEWITHSUNDEEP
perlstringsubstitution
thkala
thkala

Reputation: 86333

How to substitute arbitrary fixed strings in Perl

I want to replace a fixed string within another string using Perl. Both strings are contained in variables.

If it was impossible for the replaced string to contain any regex meta-characters, I could do something like this:

my $text = 'The quick brown fox jumps over the lazy dog!';
my $search = 'lazy';
my $replace = 'drowsy';

$text =~ s/$search/$replace/;

Alas, I want this to work for arbitrary fixed strings. E.g., this should leave $text unchanged:

my $text = 'The quick brown fox jumps over the lazy dog!';
my $search = 'dog.';
my $replace = 'donkey.';

$text =~ s/$search/$replace/;

Instead, this replaces dog! with donkey., since the dot matches the exclamation mark.

Assuming that the variable contents themselves are not hardcoded, e.g., they can come from a file or from the command line, is there a way to quote or otherwise markdown the contents of a variable so that they are not interpreted as a regular expression in such substitution operations?

Or is there a better way to handle fixed strings? Preferably something that would still allow me to use regex-like features such as anchors or back-references.

Upvotes: 2

Views: 1704

Answers (3)

Sam Choukri
Sam Choukri

Reputation: 1904

Wrap your search string with \Q...\E, which quotes any meta characters within.

$text =~ s/\Q$search\E/$replace/;

Upvotes: 6

Ry-
Ry-

Reputation: 224886

#Replace a string without using RegExp.
sub str_replace {
    my $replace_this = shift;
    my $with_this  = shift; 
    my $string   = shift;

    my $length = length($string);
    my $target = length($replace_this);

    for(my $i=0; $i<$length - $target + 1; $i++) {
        if(substr($string,$i,$target) eq $replace_this) {
            $string = substr($string,0,$i) . $with_this . substr($string,$i+$target);
            return $string; #Comment this if you what a global replace
        }
    }
    return $string;
}

Upvotes: 0

CanSpice
CanSpice

Reputation: 35788

Run your $search through quotemeta:

my $text = 'The quick brown fox jumps over the lazy dog!'; 
my $search = quotemeta('dog.'); 
my $replace = 'donkey.'; 

$text =~ s/$search/$replace/;

This will unfortunately not allow you to use other regex features. If you have a select set of features you want to escape out, perhaps you can just run your $search through a first "cleaning" regex or function, something like:

my $search = 'dog.';
$search = clean($search);

sub clean {
  my $str = shift;
  $str =~ s/\./\\\./g;
  return $str;
}

Upvotes: 7

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