Python regex - r prefix

Some may wonder why the following regex works, even without the r prefix:

import re
print (re.sub('\s ', ' ', 'hello     there      there'))

It is generally thought that the r prefix is required whenever escape sequences are used, but this example seems to contradict that.

Escape sequences and raw strings

The r prefix is used to create a "raw string", which means that the escape sequences in the string will not be interpreted as special characters. This can be useful when you want to use a character that is normally interpreted as an escape sequence, such as the backslash (\).

In the example above, the escape sequence \s is used to match one or more whitespace characters. Without the r prefix, this escape sequence would be interpreted as a tab character (\t). However, because the r prefix is used, the escape sequence is interpreted literally and matches one or more whitespace characters.

Exceptions to the rule

There are a few exceptions to the rule that escape sequences must be used with the r prefix. One exception is the \n escape sequence, which is used to represent a newline character. This escape sequence can be used without the r prefix, as shown in the following example:

print '\n'

Another exception is the \\ escape sequence, which is used to represent a backslash character. This escape sequence can also be used without the r prefix, as shown in the following example:

print '\\'

Conclusion

The r prefix is not always required when using escape sequences in Python. However, it is generally a good idea to use the r prefix to avoid any confusion or unexpected behavior.