php - PHP Regex throwing error messages

Question

I have looked at the documentation that PHP provides on PCRE Patterns. I am using a third party plugin to handle some text from the user, and the following preg_replace is failing because of missing terminating ] char. (preg_replace(): Compilation failed: missing terminating ] for character class

$input  = preg_replace('/[\]/i','',$userInput);

From what I can see the terminating delimiter is / with a character class that only has a \ in it. The i, if I can read correctly tells the expression to not care about upper or lower case. I see both opening [ and closing ].

Why is it throwing the error? What is the preg_replace trying to do?

Answer 1

You need to escape the \ otherwise it escapes the ] (and you need to escape it twice, once for the PHP string and once for PCRE).

$input  = preg_replace('/[\\\]/i','',$userInput);

And you can omit the [ and ] altogether (as well as the i).

$input  = preg_replace('/\\\/','',$userInput);

Or, you can just use str_replace:

$input  = str_replace('\\','',$userInput);