# 4.2 `re` -- Regular expression operations

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`", while `"\n"` is a one-character string containing a newline. Usually patterns will be expressed in Python code using this raw string notation.

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.

See Also:

Mastering Regular Expressions
  Book on regular expressions by Jeffrey Friedl, published by O'Reilly. The Python material in this book dates from before the `re` module, but it covers writing good regular expression patterns in great detail.

Subsections

- 4.2.1 Regular Expression Syntax
- 4.2.2 Matching vs. Searching
- 4.2.3 Module Contents
- 4.2.4 Regular Expression Objects
- 4.2.5 Match Objects