PHP - Regex replacement throws an error (preg_replace)

Question

I need to remove some weird characters in a string pulled from a source database using a regex because these characters can be located anywhere in the string and can be a little different. Here's my code right now:

$str = "LIVR DEC 20 23 $é**é$01$";
$result = preg_replace("/\$[^ ]+\$/", "", $str);

When testing it on phpliveregex.com, it works fine but when i run the code (using PHP 5.4), i get the following error:

PHP Notice: Undefined variable: é

and $result contains the original string. Why is it throwing that error?

Use single-quoted string literal to avoid string interpolation. Same with regex, use single quotes. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Aug 31, 2017 at 8:01
Thank you, replacing double quotes with single quotes in the regex did it. — BronanyX
– BronanyX, Commented Aug 31, 2017 at 8:10

Wiktor Stribiżew · Accepted Answer · 2017-08-31 08:09:26Z

You need to define the $str and the regex inside single quoted string literals. Also, since you are working with Unicode strings, you should also use the u modifier:

$str = 'LIVR DEC 20 23 $é**é$01$';
$result = preg_replace('/\$[^ ]+\$/u', "", $str);
echo $result; // => LIVR DEC 20 23

See the PHP demo.

Otherwise, when PHP engine sees "$é", it wants to interpolate it, but cannot find the variable with this name.

The regex needs single quotes because "/\$[^ ]+\$/" is equal to /$[^ ]+$/ where $ are end of string anchors, and there can't be two different end-of-string positions inside a single string.

Or, if you need to use string iterpolation and use double quoted string literals, escape $ to denote a literal $ in the string, and escape the $ inside the regex with \\\, triple backslashes (one escapes $ for the PHP engine, and \\ form a literal \ that makes an escape symbol for regex special chars):

$str = "LIVR DEC 20 23 \$é**é$01\$";
$result = preg_replace("/\\\$[^ ]+\\\$/u", "", $str);
echo $result; // => LIVR DEC 20 23

See another PHP demo.

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