fileone = open('C:/FTP.log')
for i in fileone.readlines():
    if 'on remote host' in i:
        regular = re.sub(r'\w+//ABC', 'ABC', i)
        files.append(regular)
        print files
fileone.close()

>>> import re
>>> 
>>> s = '05/08/2015 16:40:35.108 GMT+04:00INFORMATIVE: File С:\example\ABC123 has been sent via FTP to /FTPdir/ABC123 on remote host 127.0.0.1'
>>> 
>>> def f(s):
...     pat = r'File (.*\\)?(.*) has been sent'
...     match = re.search(pat, s)
...     return match and match.group(2)
... 
>>> f(s)
'ABC123'
>>>