Php validation(no spaces and special chr's)

PHP
1

please i want to know how to make a validation using php in the way that only characters(A-Z, a-z) , number and _ are allowed to be used

special characters and spaces are not allowed (will return error message)

can anyone help me please

2 

if (preg_match('#^[a-zA-Z0-9_]{4,10}$D#', $str))

I also added it must be between 4 to 10 characters.

3

thanks this is wright it works.