regex: checking for limits and character requirements

[fivestringsurf]

using php5 preg_match function

 

I am trying to limit some string data  to [uppercase,lowercase,number, underscore]only  but the string should be 7-20 characters long and Must include at least 1 uppercase 1 lowercase and 1 number.

 

How can I tighten up this code and make it leaner? ...I'm sure I probably only need to run the regex once for the check not twice like I am doing now.  Thank you

 

if(!preg_match('/^[a-zA-Z0-9_]{7,20}$/', $val) ){
echo 'no good';
}else{
      if(!preg_match('/.*^(?=.*[a-z])(?=.*[A-Z])(?=.*[0-9]).*$/', $val)){
          echo 'no good 2';
     }
}