import re
myString = '''fontSize="12px" and marginLeft="19px" and paddingTop="10px"'''
print(re.sub(r'(\d+)px', lambda m: "{}px".format(int(m.group(1)) / 2), myString))
aW1wb3J0IHJlCm15U3RyaW5nID0gJycnZm9udFNpemU9IjEycHgiIGFuZCBtYXJnaW5MZWZ0PSIxOXB4IiBhbmQgcGFkZGluZ1RvcD0iMTBweCInJycKcHJpbnQocmUuc3ViKHInKFxkKylweCcsIGxhbWJkYSBtOiAie31weCIuZm9ybWF0KGludChtLmdyb3VwKDEpKSAvIDIpLCBteVN0cmluZykp