What does preceding a string literal with “r” mean?

I first saw it used in building regular expressions across multiple lines as a method argument to re.compile(), so I assumed that r stands for RegEx.

For example:

regex = re.compile(
    r'^[A-Z]'
    r'[A-Z0-9-]'
    r'[A-Z]$', re.IGNORECASE
)

So what does r mean in this case? Why do we need it?

Answers:

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Method 1

The r means that the string is to be treated as a raw string, which means all escape codes will be ignored.

For an example:

'n' will be treated as a newline character, while r'n' will be treated as the characters followed by n.

When an 'r' or 'R' prefix is present,
a character following a backslash is
included in the string without change,
and all backslashes are left in the
string. For example, the string
literal r"n" consists of two
characters: a backslash and a
lowercase 'n'. String quotes can be
escaped with a backslash, but the
backslash remains in the string; for
example, r""" is a valid string
literal consisting of two characters:
a backslash and a double quote; r""
is not a valid string literal (even a
raw string cannot end in an odd number
of backslashes). Specifically, a raw
string cannot end in a single
backslash (since the backslash would
escape the following quote character).
Note also that a single backslash
followed by a newline is interpreted
as those two characters as part of the
string, not as a line continuation.

Source: Python string literals

Method 2

It means that escapes won’t be translated. For example:

r'n'

is a string with a backslash followed by the letter n. (Without the r it would be a newline.)

b does stand for byte-string and is used in Python 3, where strings are Unicode by default. In Python 2.x strings were byte-strings by default and you’d use u to indicate Unicode.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0