How to remove non-alphanumeric characters?

I need to remove all characters from a string which aren't in a-z A-Z 0-9 set or are not spaces.

Does anyone have a function to do this?

Sounds like you almost knew what you wanted to do already, you basically defined it as a regex.

preg_replace("/[^A-Za-z0-9 ]/", '', $string);

For unicode characters, it is :

preg_replace("/[^[:alnum:][:space:]]/u", '', $string);

Regular expression is your answer.

$str = preg_replace('/[^a-z\d ]/i', '', $str);

• The i stands for case insensitive.
• ^ means, does not start with.
• \d matches any digit.
• a-z matches all characters between a and z. Because of the i parameter you don't have to specify a-z and A-Z.
• After \d there is a space, so spaces are allowed in this regex.

If you need to support other languages, instead of the typical A-Z, you can use the following:

preg_replace('/[^\p{L}\p{N} ]+/', '', $string);

• [^\p{L}\p{N} ] defines a negated (It will match a character that is not defined) character class of:
  • \p{L}: a letter from any language.
  • \p{N}: a numeric character in any script.
  • : a space character.
• + greedily matches the character class between 1 and unlimited times.

This will preserve letters and numbers from other languages and scripts as well as A-Z:

preg_replace('/[^\p{L}\p{N} ]+/', '', 'hello-world'); // helloworld
preg_replace('/[^\p{L}\p{N} ]+/', '', 'abc@~#123-+=öäå'); // abc123öäå
preg_replace('/[^\p{L}\p{N} ]+/', '', '你好世界!@£$%^&*()'); // 你好世界

^{Note: This is a very old, but still relevant question. I am answering purely to provide supplementary information that may be useful to future visitors.}

here's a really simple regex for that:

\W|_

and used as you need it (with a forward / slash delimiter).

preg_replace("/\W|_/", '', $string);

Test it here with this great tool that explains what the regex is doing:

http://www.regexr.com/