Why it doesn't work in Python regular expression

Question

str = 'an simple example'
print re.sub(r'^AN', 'A', str, re.I)

It was expected print A simple example
but, it still print an simple example

python version 2.7(Fedora x84_64)

Answer 1

The last parameter of re.sub is a count of the replacements to do. If you want to replace with options, compile the regex:

>>> r = re.compile(r'^AN', re.IGNORECASE)
>>> my_str = 'AN simple example'
>>> r.sub('A', my_str)
'A simple example'

Answer 2

As bereal pointed out, the third parameter is the count of replacements to do.

The whole definition of re.sub is

def sub(pattern, repl, string, count=0, flags=0):

So instead of compiling the regex, you can use

print re.sub(r'^AN', 'A', str, 0, re.I)

or with a named parameter:

print re.sub(r'^AN', 'A', str, flags=re.I)

---

BTW, str is already a python function (but no reserved keyword), so redefining str could lead to strange problems.

Answer 3

If you want insensitive substitution use the insensitive option:

str = 'an simple example'
print re.sub(r'(?i)an', 'A', str)