import re

xml = """<?xml version="1.0" encoding="utf-8" ?>
<book_list>
    <book rbr="1" >
        <title> Yacc </title>
        <author> Filip Maric </author>
        <year> 2004 </year>
        <publisher> Matematicki fakultet </publisher>
        <price currency="din"> 100 </price>
    </book>
    <book rbr="2" >
        <author> Fredrik Lundh </author>
        <price currency="eur"> 50 </price>
        <publisher> O’Reilly & Associates </publisher>
        <year> 2001 </year>
        <title> Python Standard Library </title>
    </book>
</book_list>"""

m = re.search(r'<book\s*rbr="\d+"\s*>(?!.*</book>.*<title> Python Standard Library </title>).*(?P<book><title> Python Standard Library </title>).*?</book>', xml, flags=re.DOTALL)
print(m.group('book'))
m = re.search(r'<book\s*rbr="\d+"\s*>(?!.*</book>.*<title> Yacc </title>).*(?P<book><title> Yacc </title>).*?</book>', xml, flags=re.DOTALL)
print(m.group('book'))