php - Interpret escape characters in single quoted string

Question

Having a single-quoted string:

$content = 'This variable is not set by me.
Cannot do anything about it.
';

I would like to inerpret/process the string as if it was double-quoted. In other words I would like to replace all the possible escape characters (not just tab and linefeed as in this example) with the real values, taking into account that backslash might be escaped as well, thus '\n' needs to be replaced by ' '. eval() would easily do what I need but I cannot use it.

Is there some simple solution?

(A similar thread that I found deals with expansion of variables in the single-quoted string while I'm after replacing escape characters.)

Answer 1

There is a very simple way to do this, based on preg_replace^Doc and stripcslashes, both build in:

preg_replace_callback(
    '/\\([nrtvf\\$"]|[0-7]{1,3}|x[0-9A-Fa-f]{1,2})/',
    fn($matches) => stripcslashes($matches[0]), $content
);

This works as long as "\n" should become " " and the like. Demo

If you're looking for processing these strings literally, see my previous answer.

Edit: You asked in a comment:

I'm just a bit puzzled what's the difference between the output of this and stripcslashes() directly [?]

The difference is not always visible, but there is one: stripcslashes will remove the chracter if no escape sequence follows. In PHP strings, the slash is not be dropped in that case. An example, "d", d is not a special character, so PHP preserves the slash:

$content = 'd';
$content; # d
stripcslashes($content); # d
preg_replace(..., $content); # d

That's why preg_replace is useful here, it will only apply the function on those substrings where stripcslashes works as intended: all valid escape sequences.

---

After a couple of years the answer is updated for PHP 7.4+.

The original answer did contain a Demo with using the e (eval) modifier in the regex. For (mostly good) reasons it has been removed from PHP and refuses to work spilling an error like:

PHP Warning: preg_replace(): The /e modifier is no longer supported, use preg_replace_callback

In case the new version gives syntax errors (e.g. PHP < 7.4) or because of preferrence, replace the arrow function with an anonymous one like:

static function (array $matches): string {
    return stripcslashes($matches[0]);
}

Please see Replace preg_replace() e modifier with preg_replace_callback for more on-site Q&A resources on the topic to replace the e modifier in general, it was deprecated in PHP 5.5.0:

[The] e (PREG_REPLACE_EVAL) [...] was DEPRECATED in PHP 5.5.0 (Jun 2013), and REMOVED as of PHP 7.0.0 (Dec 2015).

from the PHP manual