[php] PHP preg_match - only allow alphanumeric strings and - _ characters

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
}

\w\- is probably the best but here just another alternative
Use [:alnum:]

if(!preg_match("/[^[:alnum:]\-_]/",$str)) echo "valid";

demo1 | demo2

Here is one equivalent of the accepted answer for the UTF-8 world.

if (!preg_match('/^[\p{L}\p{N}_-]+$/u', $string)){
  //Disallowed Character In $string
}

Explanation:

• [] => character class definition
• p{L} => matches any kind of letter character from any language
• p{N} => matches any kind of numeric character
• _- => matches underscore and hyphen
• + => Quantifier — Matches between one to unlimited times (greedy)
• /u => Unicode modifier. Pattern strings are treated as UTF-16. Also causes escape sequences to match unicode characters

Note, that if the hyphen is the last character in the class definition it does not need to be escaped. If the dash appears elsewhere in the class definition it needs to be escaped, as it will be seen as a range character rather then a hyphen.

if(!preg_match('/^[\w-]+$/', $string1)) {
   echo "String 1 not acceptable acceptable";
   // String2 acceptable
}

Why to use regex? PHP has some built in functionality to do that

<?php
    $valid_symbols = array('-', '_');
    $string1 = "This is a string*";
    $string2 = "this_is-a-string";

    if(preg_match('/\s/',$string1) || !ctype_alnum(str_replace($valid_symbols, '', $string1))) {
        echo "String 1 not acceptable acceptable";
    }
?>

preg_match('/\s/',$username) will check for blank space

!ctype_alnum(str_replace($valid_symbols, '', $string1)) will check for valid_symbols