regex - PHP preg_match - only allow alphanumeric strings and - _ characters

Question

I need the regex to check if a string only contains numbers, letters, hyphens or underscore

$string1 = "This is a string*";
$string2 = "this_is-a-string";

if(preg_match('******', $string1){
   echo "String 1 not acceptable acceptable";
   // String2 acceptable
}

Answer 1

Code:

if(preg_match('/[^a-z_-0-9]/i', $string))
{
  echo "not valid string";
}

Explanation:

• [] => character class definition
• ^ => negate the class
• a-z => chars from 'a' to 'z'
• _ => underscore
• - => hyphen '-' (You need to escape it)
• 0-9 => numbers (from zero to nine)

The 'i' modifier at the end of the regex is for 'case-insensitive' if you don't put that you will need to add the upper case characters in the code before by doing A-Z