use warnings;
use strict;

sub interpolate {
	local $_ = shift;
	my $vars = shift;
	my @out;
	pos=undef;
	while (1) {
		if ( m{\G ( [^\\\$]+ ) }xmsgc ) { push @out, $1 }
		elsif ( m{\G \$ (?| (\w+) | \{(\w+)\} ) }xmsgc )
			{ push @out, $vars->{$1} }
		elsif ( m{\G \\ (.) }xmsgc ) {
			$1 eq "\\" or $1 eq "\$"
				or die "unexpected backslash escape";
			push @out, $1;
		}
		else { last }
	}
	die "parse of '$_' failed at pos ".(pos//0)
		if !defined pos || pos!=length;
	return join '', @out;
}

use Test::More;
sub exception (&) { eval { shift->(); 1 } ? undef : ($@ || die) }

my %env = ( USER => "foobar", quz => "\$baz \\\\ \\\$" );

is interpolate("USER", \%env), "USER";
like exception { interpolate("\\USER"), \%env },
	qr/\bunexpected backslash\b/;
is interpolate("\\\\USER", \%env), "\\USER";
is interpolate("\$USER", \%env), "foobar";
is interpolate("\${USER}", \%env), "foobar";
is interpolate("\\\$USER", \%env), "\$USER";
is interpolate("\\\\\$USER", \%env), "\\foobar";
is interpolate("\\\\\\\$USER", \%env), "\\\$USER";
is interpolate("\\\\\\\\\$USER", \%env), "\\\\foobar";
is interpolate("\\\\\\\\\${USER}", \%env), "\\\\foobar";
is interpolate("\$quz", \%env), "\$baz \\\\ \\\$";
like exception { interpolate("USER\$"), \%env },
	qr/\bparse of 'USER\$' failed\b/;
like exception { interpolate("\${USER"), \%env },
	qr/\bparse of '\$\{USER' failed\b/;

done_testing;

##</code><code>##

use warnings;
use strict;
use Test::More;
sub exception (&) { eval { shift->(); 1 } ? undef : ($@ || die) }

my %_interp_map = ( '\\'=>'\\', '$'=>'$' ); # or use String::Unescape
sub interpolate {
	my $str = shift;
	$str =~ s{ (?<!\\) ((?:\\\\)*) \$ (?| (\w+) | \{(\w+)\} ) }
		{ $1.$ENV{$2} }xeg;
	$str =~ s{ \\ (.) }
		{ $_interp_map{$1} || die "unexpected backslash escape" }xeg;
	return $str;
}

local $ENV{USER} = "foobar";

is interpolate("USER"), "USER";
like exception { interpolate("\\USER") }, qr/unexpected backslash/i;
is interpolate("\\\\USER"), "\\USER";
is interpolate("\$USER"), "foobar";
is interpolate("\${USER}"), "foobar";
is interpolate("\\\$USER"), "\$USER";
is interpolate("\\\\\$USER"), "\\foobar";
is interpolate("\\\\\\\$USER"), "\\\$USER";
is interpolate("\\\\\\\\\$USER"), "\\\\foobar";
is interpolate("\\\\\\\\\${USER}"), "\\\\foobar";

done_testing;