php code to validate alphanumeric string

Try this

// Validate alphanumeric
if (preg_match('/^[a-zA-Z]+[a-zA-Z0-9._]+$/', $input)) {
    // Valid
} else {
    // Invalid
}

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

Use ctype_alnum like below:

if(ctype_alnum($string)){
    echo "Yes, It's an alphanumeric string/text";
}
else{
    echo "No, It's not an alphanumeric string/text";
}

Read function specification on php.net