Question

how could I remove all characters from a string that aren’t a-z/A-Z and 0-9 and _ with PHP?
I tried that but it has no effect, it returns the same string back:

preg_replace('[^a-zA-Z0-9_]', '', 'Testdo123_-:=)§%&');

Answer 1

The preg_ prefixed functions require PCRE styled regular expression that use delimiters to separate the regular expression from optional flags/modifiers.

But you forgot delimiters. Or, to be precise: PHP takes the [ and ] as delimiters, leaving just ^a-zA-Z0-9_ as your actual regular expression.

So try this (using / as delimiters):

preg_replace('/[^a-zA-Z0-9_]/', '', 'Testdo123_-:=)§%&')