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
• az => 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 AZ

Answer 2

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

Answer 3

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.

Answer 4

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

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

demo1 | demo2

Answer 5

Here is a funky non-regex method using str_word_count() :

if($string===str_word_count($string,1,'-_0...9')[0]){
//                                     ^^^^^^^--- characters to allow, see documentation 
    echo "pass";
}else{
    echo "fail";
}

View the Demo Link where I show how different inputs result in different output arrays.

Answer 6

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