[RESOLVED] Expanding escape sequences in existing string, without eval?

[evan_k]

I have an existing string with escape sequences in them, and I want to expand them, but I don't want to run it through eval because I don't want any variables to be interpolated. Is there a simple way to do this by using PHP's existing ability to find and expand escape sequences, but that will not interpret variables?

<?php

// want to interpolate *any* backslash escape sequences
$str = 'Quantity:\t500\nPrice:\t$5.25 each';

// this interpolates $5 as a variable, which I don't want
eval('$str = "' . $str . '";');

[laserlight]

It sounds like you want to use double quote delimited strings instead. Such strings recognise more escape sequences, and even parse variables embedded within them.

$str = "Quantity:\t500\nPrice:\t$5.25 each";

[evan_k]

That's exactly what I don't want, though...I want to avoid variable interpolation. That's my snafu.

[laserlight]

Oh, sorry, I did not read that part carefully.

The answer then is no. You have to perform the string parsing yourself, e.g., by replacing '\t' with "\t".

[bradgrafelman]

This seems like a weird way to do it, but this appears to work:

$variable = "foobar";
$string = 'This is a test! A line break:\n and a \t tab! no $variable tho!';

$pattern = '/(?<!\\\)(\\\(?:n|r|t|v|f|[0-7]{1,3}|x[0-9a-f]{1,2}))/ie';
$replace = 'eval(\'return "$1";\')';

echo preg_replace($pattern, $replace, $string);

Output:

This is a test! A line break:
 and a 	 tab! no $variable tho!

Note that if you didn't care about the special escape sequences and only wanted to expand \r, \n, \t, and \v, you could simply do:

$string = str_replace(array('\n', '\r', '\t', '\v'), array("\n", "\r", "\t", "\v"), $string);

[evan_k]

Hmm, once I sat down to tackle this problem, it didn't get nearly as hairy as I expected it to be. After some muddling through the online docs, and many, many obscenities, I produced this, which seems quite workable, and doesn't present any security risks that I can see:

<?php

// where we do all our magic
function expand_escape($string) {
	return preg_replace_callback(
		'/\\\([nrtvf]|[0-7]{1,3}|x[0-9A-Fa-f]{1,2})?/',
		create_function(
			'$matches',
			'return ($matches[0] == "\\\\") ? "" : eval( sprintf(\'return "%s";\', $matches[0]) );'
		),
		$string
	);
}

// a string to test, and show the before and after
$before = 'Quantity:\t500\nPrice:\t$5.25 each';
$after = expand_escape($before);
var_dump($before, $after);

/* Outputs:
string(34) "Quantity:\t500\nPrice:\t$5.25 each"
string(31) "Quantity:	500
Price:	$5.25 each"
*/

?>

edit: heh, I just noticed Brad's solution after I posted my own. I need to remember to refresh the page before I follow up, I guess. At any rate, his seems to be a little cleaner, as it doesnt require the lambda function. i also forgot the 'x' for hex values, though I removed the 'i' flag and added 'A-F' to the range, since \N, \R, \T, etc aren't valid escape sequences. this is the final function I ended up with (thanks brad):

<?php
function expand_escape($string) {
	return preg_replace(
		'/(?<!\\\)(\\\(?:[nrtvf]|[0-7]{1,3}|x[0-9A-Fa-f]{1,2}))/e',
		'eval(\'return "$1";\')',
		$string
	);
}

[Weedpacket]

evan_k wrote:

but I don't want to run it through eval because I don't want any variables to be interpolated.

Well, you can stop that easily enough...

// want to interpolate *any* backslash escape sequences
$str = 'Quantity:\t500\nPrice:\t$5.25 each';

$str = str_replace('$','\\$', $str);

eval('$str = "' . $str . '";'); 

[bradgrafelman]

I'd thought about simply doing that, though you'd also want to escape any single quote within the string so that it doesn't break the eval() code.

At any rate, guess I just wanted to create yet another regexp expression to solve a problem... heh. :o