PHP regular expression - filter number only

To remove anything that is not a number:

$output = preg_replace('/[^0-9]/', '', $input);

Explanation:

• [0-9] matches any number between 0 and 9 inclusively.
• ^ negates a [] pattern.
• So, [^0-9] matches anything that is not a number, and since we're using preg_replace, they will be replaced by nothing '' (second argument of preg_replace).

Using is_numeric or intval is likely the best way to validate a number here, but to answer your question you could try using preg_replace instead. This example removes all non-numeric characters:

$output = preg_replace( '/[^0-9]/', '', $string );