How can I manually interpolate string escapes in a Perl string?

Question

In perl suppose I have a string like 'hello\tworld\n', and what I want is:

'hello  world
'

That is, "hello", then a literal tab character, then "world", then a literal newline. Or equivalently, "hello\tworld\n" (note the double quotes).

In other words, is there a function for taking a string with escape sequences and returning an equivalent string with all the escape sequences interpolated? I don't want to interpolate variables or anything else, just escape sequences like \x, where x is a letter.

FMc · Accepted Answer · 2010-04-17 21:51:21Z

8

Sounds like a problem that someone else would have solved already. I've never used the module, but it looks useful:

use String::Escape qw(unbackslash);
my $s = unbackslash('hello\tworld\n');

answered Apr 17, 2010 at 21:51

FMc

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2 Comments

Ryan C. Thompson Over a year ago

Thanks very much for this. I found String::Interpolate, but that does something completely different.

Ryan C. Thompson Over a year ago

Also, for people using this in Ubuntu 9.10, the version of String::Escape doesn't have unbackslash. Instead it has unprintable.

Snowhare · Accepted Answer · 2010-04-17 22:01:18Z

2

You can do it with 'eval':

my $string = 'hello\tworld\n';
my $decoded_string = eval "\"$string\"";

Note that there are security issues tied to that approach if you don't have 100% control of the input string.

Edit: If you want to ONLY interpolate \x substitutions (and not the general case of 'anything Perl would interpolate in a quoted string') you could do this:

my $string = 'hello\tworld\n';
$string =~ s#([^\\A-Za-z_0-9])#\\$1#gs;
my $decoded_string = eval "\"$string\"";

That does almost the same thing as quotemeta - but exempts '\' characters from being escaped.

Edit2: This still isn't 100% safe because if the last character is a '\' - it will 'leak' past the end of the string though...

Personally, if I wanted to be 100% safe I would make a hash with the subs I specifically wanted and use a regex substitution instead of an eval:

my %sub_strings = (
    '\n' => "\n",
    '\t' => "\t",
    '\r' => "\r",
);

$string =~ s/(\\n|\\t|\\n)/$sub_strings{$1}/gs;

edited Apr 17, 2010 at 22:01

answered Apr 17, 2010 at 21:29

Snowhare

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3 Comments

Ryan C. Thompson Over a year ago

Can I use a combination of eval and quotemeta to do this safely? Basically, (my $interpolated_string = eval(q(") . quotemeta($string) . q("))) =~ s/\\(.)/\1/g.

Snowhare Over a year ago

Not without it killing the interpolation as well. ;)

Ryan C. Thompson Over a year ago

Well, I only want to interpolate \x, where x is any letter. Not variables or other stuff.

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