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

Answer 1

No, as the documentation pasted in explains the r prefix to a string indicates that the string is a raw string.

Because of the collisions between Python escaping of characters and regex escaping, both of which use the back-slash \ character, raw strings provide a way to indicate to python that you want an unescaped string.

Examine the following:

>>> "\n"
'\n'
>>> r"\n"
'\\n'
>>> print "\n"


>>> print r"\n"
\n

Prefixing with an r merely indicates to the string that backslashes \ should be treated literally and not as escape characters for python.

This is helpful, when for example you are searching on a word boundry. The regex for this is \b, however to capture this in a Python string, I'd need to use "\\b" as the pattern. Instead, I can use the raw string: r"\b" to pattern match on.

This becomes especially handy when trying to find a literal backslash in regex. To match a backslash in regex I need to use the pattern \\, to escape this in python means I need to escape each slash and the pattern becomes "\\\\", or the much simpler r"\\".

As you can guess in longer and more complex regexes, the extra slashes can get confusing, so raw strings are generally considered the way to go.