只想在字符串中允许某些特定字符

Question

How can I only allow the characters W, L, and D in a string? I have written this

if ((strpos($input, 'w') === false) 
 && (strpos($input, 'd') === false) 
 && (strpos($input, 'l') === false)) { 

 $answer = "FALSE";

} else {

 $answer = "TRUE";

}

but this only works if all the characters are W, L, D or none of the characters are W, L or D. I cant work out how to get it to spot when just some of the characters are incorrect.

eg

wwwlld = TRUE (correct)
12aab6 = FALSE (correct)
www44d = TRUE (incorrect, this needs to be FALSE as it contains '44')

Any help much appreciated.

P

Answer 1

This is a classic example of requiring regular expressions, which are the preg_match function in PHP.

You probably want to use something like the following

$pattern = '/^[wdl]*$/';

This would only match a string where between the start and the end of the string, 0 or more characters in the set w, d or l.