Converting Posix Regex to PCRE in PHP

Question

I have a cms that on the front-end grabs pages by id, as shown here:

$pageid = ereg_replace("[^0-9]", "", $_GET['pid']);

This is some old code I am trying to update since POSIX is deprecated, however my efforts to convert (using preg_replace) have been unsuccessful. If anyone could convert that line for me, I would greatly appreciate it.

Add code from comments

My first guess was something along the lines of

$pageid = preg_replace("/[^0-9]/","",$_GET['pid']; 

Which gave errors, so I further reduced it to

$pageid = preg_replace("/^0-9/","",$_GET['pid'] 

Forgive me, my understanding of regex is rather limited.

Answer 1

Let's explain what the posix pattern does.

[^0-9]

[ is the start of a character class and ] is the end. When the character class starts with a ^ it means it's inverted (= matches everything but what is listed). 0-9 is all digits.

So globally [^0-9] matches everything which is NOT a digit.

The same pattern is also available in PCRE so this will work:

$page_id = preg_replace('/[^0-9]/', '', $_GET['pid']);

PCRE has some nice shortcuts to express things. For example [0-9] can be replaced with \d (d stands for digits) Also \D (using a capital letter) is the inverse and thus equivalent to [^0-9].

Which leads to the following:

$page_id = preg_replace('/\D/', '', $_GET['pid']);