Your question is more or less a duplicate of this question (just answered it).
Think in these terms:
- if your element has attributes and/or nested elements, it must be a complex type.
- if your element can have text, but no markup (nested elements) then it must be of simpleContent
- simpleContent can only be extended from a simple type (if you need to add an attribute). If you need additional constraints on the simple type, you need to specifically create a new simple type, restricting the base type, then extend that in a simple content.
This post on SO shows you an end-to-end example, of the above explanation. For your particular case:
<?xml version="1.0" encoding="utf-8" ?>
<!-- XML Schema generated by QTAssistant/XSD Module (http://www.paschidev.com) -->
<xsd:schema elementFormDefault="qualified" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:simpleType name="tSomeRestrictedText">
<xsd:restriction base="xsd:string">
<xsd:pattern value="AAA"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:element name="some_element">
<xsd:complexType>
<xsd:simpleContent>
<xsd:extension base="tSomeRestrictedText">
<xsd:attribute name="my_attr" type="xsd:string"/>
</xsd:extension>
</xsd:simpleContent>
</xsd:complexType>
</xsd:element>
</xsd:schema>