How can I do multiple substitutions using regex in python?

The answer proposed by @nhahtdh is valid, but I would argue less pythonic than the canonical example, which uses code less opaque than his regex manipulations and takes advantage of python's built-in data structures and anonymous function feature.

A dictionary of translations makes sense in this context. In fact, that's how the Python Cookbook does it, as shown in this example (copied from ActiveState http://code.activestate.com/recipes/81330-single-pass-multiple-replace/ )

import re 

def multiple_replace(dict, text):
  # Create a regular expression  from the dictionary keys
  regex = re.compile("(%s)" % "|".join(map(re.escape, dict.keys())))

  # For each match, look-up corresponding value in dictionary
  return regex.sub(lambda mo: dict[mo.string[mo.start():mo.end()]], text) 

if __name__ == "__main__": 

  text = "Larry Wall is the creator of Perl"

  dict = {
    "Larry Wall" : "Guido van Rossum",
    "creator" : "Benevolent Dictator for Life",
    "Perl" : "Python",
  } 

  print multiple_replace(dict, text)

So in your case, you could make a dict trans = {"a": "aa", "b": "bb"} and then pass it into multiple_replace along with the text you want translated. Basically all that function is doing is creating one huge regex containing all of your regexes to translate, then when one is found, passing a lambda function to regex.sub to perform the translation dictionary lookup.

You could use this function while reading from your file, for example:

with open("notes.txt") as text:
    new_text = multiple_replace(replacements, text.read())
with open("notes2.txt", "w") as result:
    result.write(new_text)

I've actually used this exact method in production, in a case where I needed to translate the months of the year from Czech into English for a web scraping task.

As @nhahtdh pointed out, one downside to this approach is that it is not prefix-free: dictionary keys that are prefixes of other dictionary keys will cause the method to break.

You can use capturing group and backreference:

re.sub(r"([characters])", r"\1\1", text.read())

Put characters that you want to double up in between []. For the case of lower case a, b, c:

re.sub(r"([abc])", r"\1\1", text.read())

In the replacement string, you can refer to whatever matched by a capturing group () with \n notation where n is some positive integer (0 excluded). \1 refers to the first capturing group. There is another notation \g<n> where n can be any non-negative integer (0 allowed); \g<0> will refer to the whole text matched by the expression.

---

If you want to double up all characters except new line:

re.sub(r"(.)", r"\1\1", text.read())

---

If you want to double up all characters (new line included):

re.sub(r"(.)", r"\1\1", text.read(), 0, re.S)

Using tips from how to make a 'stringy' class, we can make an object identical to a string but for an extra sub method:

import re
class Substitutable(str):
  def __new__(cls, *args, **kwargs):
    newobj = str.__new__(cls, *args, **kwargs)
    newobj.sub = lambda fro,to: Substitutable(re.sub(fro, to, newobj))
    return newobj

This allows to use the builder pattern, which looks nicer, but works only for a pre-determined number of substitutions. If you use it in a loop, there is no point creating an extra class anymore. E.g.

>>> h = Substitutable('horse')
>>> h
'horse'
>>> h.sub('h', 'f')
'forse'
>>> h.sub('h', 'f').sub('f','h')
'horse'