This module provides regular expression matching operations similar to
those found in Perl. Regular expression pattern strings may not
contain null bytes, but can specify the null byte using the
\number notation. Both patterns and strings to be
searched can be Unicode strings as well as 8-bit strings. The
re module is always available.
Regular expressions use the backslash character ("\") to
indicate special forms or to allow special characters to be used
without invoking their special meaning. This collides with Python's
usage of the same character for the same purpose in string literals;
for example, to match a literal backslash, one might have to write
'\\\\' as the pattern string, because the regular expression
must be "\\", and each backslash must be expressed as
"\\" inside a regular Python string literal.
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",
"\n" is a one-character string containing a newline.
Usually patterns will be expressed in Python code using this raw
Implementation note: The re module has two distinct implementations: sre is the default implementation and includes Unicode support, but may run into stack limitations for some patterns. Though this will be fixed for a future release of Python, the older implementation (without Unicode support) is still available as the pre module.