Hi I need to validate password for only alphanumeric chars( should present both alphabets and numbers from 0..9) . I mean it should contain at least one digit and remaining alphabets

The regex I use is like this : 

```
     <xsd:simpleType name="PasswordType">
         <xsd:restriction base="xsd:string">
             <xsd:pattern value="?!^[0-9]*$)(?!^[a-zA-Z]*$)^([a-zA-Z0-9]{8,20})$"/>
         </xsd:restriction>
    </xsd:simpleType>

```

but eclipse complains the error :
 *InvalidRegex: Pattern value '?!^[0-9]*$)(?!^[a-zA-Z]*$)^([a-zA-Z0-9]{8,20})$' is not a valid regular expression. The reported error was: 
 'Unexpected meta character.'.*

Could anybody help how to solve this issue?

Thanks