Question

What does the # sign do differently than the /?

$output = preg_replace('#[^A-Za-z0-9]#i', '', $input);
$output = preg_replace('/[^A-Za-z0-9]/i', '', $input); 

And what does the letter i do after /[^A-Za-z0-9]/?

Also what does the ^ mean?

Answer 1

In some languages, it does not matter what type of character starts or ends the pattern portion of the regular expression, so long as it is the same at the beginning and the end (I believe this is a holdover from Perl, arguably the first great regex language). Since PHP follows this line of thought, # and / are equivalent.

• i = “Make this search case insensitive”
• [^...] = exclude everything between the square brackets (^ basically means “exclusion” in this context).

You can learn a lot about regular expressions here.