I want a regular expression to validate a nickname: 6 to 36 characters, it should contain at least one letter. Other allowed characters: 0-9 and underscores.

This is what I have now:

if(!preg_match('/^.*(?=\d{0,})(?=[a-zA-Z]{1,})(?=[a-zA-Z0-9_]{6,36}).*$/i', $value)){
 echo 'bad';
}
else{
 echo 'good';
}

This seems to work, but when a validate this strings for example:

11111111111a > is not valid, but it should aaaaaaa!aaaa > is valid, but it shouldn't

Any ideas to make this regexp better?