How do I search a particular string out of file1, and update a csv file
File1 is formatted as such:
thisismy@email.com:20110708
thisisnotmy@email.com:20110908
thisisyour@email.com:20090807
...

import re
emails = re.findall(r'^(.*@.*?):', open('filename.csv').read())

import csv

# 'b' flag for binary is necessary if on Windows otherwise crlf hilarity ensues
with open('/path/to/file1.txt','rb') as fin:
  csv_reader = csv.reader(fin, delimiter=":")
  # Header in line 1? Skip over. Otherwise no need for next line.
  csv_reader.next()
  # populate dict with email address as key and date as value
  # dictionary comprehensions supported in 2.7+
  # on a lower version? use: d = dict((line[0],line[1]) for line in csv_reader)
  email_address_dict = {line[0]: line[1] for line in csv_reader}

# there are ways to modify a file in-place
# but it's easier to write to a new file
with open('/path/to/file2.txt','rb') as fin,
     open('/path/to/file3.txt','wb') as fou:
  csv_reader = csv.reader(fin, delimiter=":")
  csv_writer = csv.writer(fou, delimiter=":")
  # Header in line 1? Skip over. Otherwise no need for next line.
  csv_writer.writerow( csv_reader.next() )
  for line in csv_reader:
    # construct new line
    # looking up date value in just-created dict
    # the new date value is inserted position 5 (zero-based)
    newline = line[0:5]
    newline.append(email_address_dict[line[0]])
    newline.extend(line[6:])
    csv_writer.writerow(newline)