Core Python Programming (2nd Edition)

You may have seen the use of raw strings in some of the examples
above. Regular expressions were a strong motivation for the advent of
raw strings. The reason is because of conflicts between ASCII characters
and regular expression special characters. As a special symbol, "\b"
represents the ASCII character for backspace, but "\b" is also a regular
expression special symbol, meaning "match" on a word boundary. In
order for the RE compiler to see the two characters "\b" as your string
and not a (single) backspace, you need to escape the backslash in the
string by using another backslash, resulting in "\\b."
This can get messy, especially if you have a lot of special characters in
your string, adding to the confusion. We were introduced to raw strings
back in Chapter 6, and they can be (and are often) used to help keep
REs looking somewhat manageable. In fact, many Python programmers
swear by these and only use raw strings when defining regular
expressions.
Here are some examples of differentiating between the backspace "\b"
and the regular expression "\b," with and without raw strings:
>>> m = re.match('\bblow', 'blow') # backspace, no match
>>> if m is not None: m.group()
...
>>> m = re.match('\\bblow', 'blow') # escaped \, now it works
>>> if m is not None: m.group()
...
'blow'
>>> m = re.match(r'\bblow', 'blow') # use raw string instead
>>> if m is not None: m.group()
...
'blow'
You may have recalled that we had no trouble using "\d" in our regular
expressions without using raw strings. That is because there is no ASCII
equivalent special character, so the regular expression compiler already
knew you meant a decimal digit.