What does the "r" in pythons re.compile(r' pattern flags') mean?

Question

I am reading through http://docs.python.org/2/library/re.html. According to this the "r" in pythons re.compile(r' pattern flags') refers the raw string notation :

The solution is to use Python’s raw string notation for regular expression patterns; backslashes are not handled in any special way in a string literal prefixed with 'r'. So r"\n" is a two-character string containing '\' and 'n', while "\n" is a one-character string containing a newline. Usually patterns will be expressed in Python code using this raw string notation.

Would it be fair to say then that:

re.compile(r pattern) means that "pattern" is a regex while, re.compile(pattern) means that "pattern" is an exact match?

Answer 1

As @PauloBu stated, the r string prefix is not specifically related to regex's, but to strings generally in Python.

Normal strings use the backslash character as an escape character for special characters (like newlines):

>>> print('this is \n a test') this is   a test 

The r prefix tells the interpreter not to do this:

>>> print(r'this is \n a test') this is \n a test >>>  

This is important in regular expressions, as you need the backslash to make it to the re module intact - in particular, \b matches empty string specifically at the start and end of a word. re expects the string \b, however normal string interpretation '\b' is converted to the ASCII backspace character, so you need to either explicitly escape the backslash ('\\b'), or tell python it is a raw string (r'\b').

>>> import re >>> re.findall('\b', 'test') # the backslash gets consumed by the python string interpreter [] >>> re.findall('\\b', 'test') # backslash is explicitly escaped and is passed through to re module ['', ''] >>> re.findall(r'\b', 'test') # often this syntax is easier ['', '']