Perl substitute strings in hash - regex

Question

Hi I have an perl hash of arbitrary depth. I want to substitute the a string in entire structure with something else.
What is the right approach to do it?
I did something like this
#convert the hash to string for manipulation
my $data = YAML::Dump(%hash);
# do manipulation
---
---
# time to get back the hash
%hash = YAML::Load($data);

Answer 1

Your idea seems very risky to me, since it can be hard to be sure that the substitution won't destroy something in the output of YAML::Dump that will prevent the result from being read back in again, or worse, something that will alter the structure of the hash as it is represented in the dump string. What if the manipulation you are trying to perform is to replace : with , or ’ with ', or something of that sort?
I would probably do something more like this:
use Scalar::Util 'reftype';
# replace $this with $that in key names and string values of $hash
# recursively apply replacement in hash and all its subhashes
sub hash_replace {
my ($hash, $this, $that) = #_;
for my $k (keys %$hash) {
# substitution in value
my $v = $hash->{$k};
if (ref $v && reftype($v) eq "HASH") {
hash_replace($v, $this, $that);
} elsif (! ref $v) {
$v =~ s/$this/$that/og;
}
my $new_hash = {};
for my $k (keys %$hash) {
# substitution in key
(my $new_key = $k) =~ s/$this/$that/og;
$new_hash->{$new_key} = $hash->{$k};
}
%$hash = %$new_hash; # replace old keys with new keys
}
The s/…/…/ replacement I used here may not be appropriate for your task; you should feel free to use something else. For example, instead of strings $this and $that you might pass two functions, $key_change and $val_change which are applied to keys and to values, respectively, returning the modified versions. See the ###### lines below:
use Scalar::Util 'reftype';
# replace $this with $that in key names and string values of $hash
# recursively apply replacement in hash and all its subhashes
sub hash_replace {
my ($hash, $key_change, $val_change) = #_;
for my $k (keys %$hash) {
# substitution in value
my $v = $hash->{$k};
if (ref $v && reftype($v) eq "HASH") {
hash_replace($v, $key_change, $val_change);
} elsif (! ref $v) {
$v = $val_change->($v); #######
}
}
my $new_hash = {};
for my $k (keys %$hash) {
# substitution in key
my $new_key = $key_change->($k); #######
$new_hash->{$new_key} = $hash->{$k};
}
%$hash = %$new_hash;
}

Answer 2

Here's one way to attack it, by recursing through the hash. In this code, you pass in a sub that does whatever you like to each value in the nested hash. This code only modifies the values, not the keys, and it ignores other reference types (ie. scalar refs, array refs) in the nested structure.
#!/usr/bin/perl -w
use Modern::Perl;
## Visit all nodes in a nested hash. Bare-bones.
sub visit_hash
{
my ($start, $sub) = #_;
my #q = ( $start );
while (#q) {
my $hash = pop #q;
foreach my $key ( keys %{$hash} ) {
my $ref = ref($hash->{$key});
if ( $ref eq "" ) { # not a reference
&$sub( $hash->{$key} );
next;
}
if ( $ref eq "HASH" ) { # reference to a nested hash
push #q, $hash->{$key};
next;
}
# ignore other reference types.
}
}
}
The following gives an example of how to use it, replacing e with E in a nested hash:
# Example of replacing a string in all values:
my %hash =
(
a => "fred",
b => "barney",
c => "wilma",
d => "betty",
nest1 =>
{
1 => "red",
2 => "orange",
3 => "green"
},
nest2 =>
{
x => "alpha",
y => "beta",
z => "gamma"
},
);
use YAML::XS;
print "Before:\n";
print Dump( \%hash );
# now replace 'e' with 'E' in all values.
visit_hash( \%hash, sub { $_[0] =~ s/e/E/g; } );
print "After:\n";
print Dump( \%hash );