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Regex HOWTO: invalid string literals result in SyntaxWarning #148092

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56 changes: 28 additions & 28 deletions Doc/howto/regex.rst
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
.. _regex-howto:

****************************
Regular Expression HOWTO
Regular expression HOWTO
****************************

:Author: A.M. Kuchling <amk@amk.ca>
Expand Down Expand Up @@ -47,7 +47,7 @@ Python code to do the processing; while Python code will be slower than an
elaborate regular expression, it will also probably be more understandable.


Simple Patterns
Simple patterns
===============

We'll start by learning about the simplest possible regular expressions. Since
Expand All @@ -59,7 +59,7 @@ expressions (deterministic and non-deterministic finite automata), you can refer
to almost any textbook on writing compilers.


Matching Characters
Matching characters
-------------------

Most letters and characters will simply match themselves. For example, the
Expand Down Expand Up @@ -159,7 +159,7 @@ match even a newline. ``.`` is often used where you want to match "any
character".


Repeating Things
Repeating things
----------------

Being able to match varying sets of characters is the first thing regular
Expand Down Expand Up @@ -210,7 +210,7 @@ this RE against the string ``'abcbd'``.
| | | ``[bcd]*`` is only matching |
| | | ``bc``. |
+------+-----------+---------------------------------+
| 6 | ``abcb`` | Try ``b`` again. This time |
| 7 | ``abcb`` | Try ``b`` again. This time |
| | | the character at the |
| | | current position is ``'b'``, so |
| | | it succeeds. |
Expand Down Expand Up @@ -255,7 +255,7 @@ is equivalent to ``+``, and ``{0,1}`` is the same as ``?``. It's better to use
to read.


Using Regular Expressions
Using regular expressions
=========================

Now that we've looked at some simple regular expressions, how do we actually use
Expand All @@ -264,7 +264,7 @@ expression engine, allowing you to compile REs into objects and then perform
matches with them.


Compiling Regular Expressions
Compiling regular expressions
-----------------------------

Regular expressions are compiled into pattern objects, which have
Expand Down Expand Up @@ -295,7 +295,7 @@ disadvantage which is the topic of the next section.

.. _the-backslash-plague:

The Backslash Plague
The backslash plague
--------------------

As stated earlier, regular expressions use the backslash character (``'\'``) to
Expand Down Expand Up @@ -335,7 +335,7 @@ expressions will often be written in Python code using this raw string notation.

In addition, special escape sequences that are valid in regular expressions,
but not valid as Python string literals, now result in a
:exc:`DeprecationWarning` and will eventually become a :exc:`SyntaxError`,
:exc:`SyntaxWarning` and will eventually become a :exc:`SyntaxError`,
which means the sequences will be invalid if raw string notation or escaping
the backslashes isn't used.

Expand All @@ -351,7 +351,7 @@ the backslashes isn't used.
+-------------------+------------------+


Performing Matches
Performing matches
------------------

Once you have an object representing a compiled regular expression, what do you
Expand All @@ -369,10 +369,10 @@ for a complete listing.
| | location where this RE matches. |
+------------------+-----------------------------------------------+
| ``findall()`` | Find all substrings where the RE matches, and |
| | returns them as a list. |
| | return them as a list. |
+------------------+-----------------------------------------------+
| ``finditer()`` | Find all substrings where the RE matches, and |
| | returns them as an :term:`iterator`. |
| | return them as an :term:`iterator`. |
+------------------+-----------------------------------------------+

:meth:`~re.Pattern.match` and :meth:`~re.Pattern.search` return ``None`` if no match can be found. If
Expand Down Expand Up @@ -473,7 +473,7 @@ Two pattern methods return all of the matches for a pattern.
The ``r`` prefix, making the literal a raw string literal, is needed in this
example because escape sequences in a normal "cooked" string literal that are
not recognized by Python, as opposed to regular expressions, now result in a
:exc:`DeprecationWarning` and will eventually become a :exc:`SyntaxError`. See
:exc:`SyntaxWarning` and will eventually become a :exc:`SyntaxError`. See
:ref:`the-backslash-plague`.

:meth:`~re.Pattern.findall` has to create the entire list before it can be returned as the
Expand All @@ -491,7 +491,7 @@ result. The :meth:`~re.Pattern.finditer` method returns a sequence of
(29, 31)


Module-Level Functions
Module-level functions
----------------------

You don't have to create a pattern object and call its methods; the
Expand All @@ -518,7 +518,7 @@ Outside of loops, there's not much difference thanks to the internal
cache.


Compilation Flags
Compilation flags
-----------------

.. currentmodule:: re
Expand Down Expand Up @@ -642,7 +642,7 @@ of each one.
whitespace is in a character class or preceded by an unescaped backslash; this
lets you organize and indent the RE more clearly. This flag also lets you put
comments within a RE that will be ignored by the engine; comments are marked by
a ``'#'`` that's neither in a character class or preceded by an unescaped
a ``'#'`` that's neither in a character class nor preceded by an unescaped
backslash.

For example, here's a RE that uses :const:`re.VERBOSE`; see how much easier it
Expand All @@ -669,7 +669,7 @@ of each one.
to understand than the version using :const:`re.VERBOSE`.


More Pattern Power
More pattern power
==================

So far we've only covered a part of the features of regular expressions. In
Expand All @@ -679,7 +679,7 @@ retrieve portions of the text that was matched.

.. _more-metacharacters:

More Metacharacters
More metacharacters
-------------------

There are some metacharacters that we haven't covered yet. Most of them will be
Expand Down Expand Up @@ -875,7 +875,7 @@ Backreferences like this aren't often useful for just searching through a string
find out that they're *very* useful when performing string substitutions.


Non-capturing and Named Groups
Non-capturing and named groups
------------------------------

Elaborate REs may use many groups, both to capture substrings of interest, and
Expand Down Expand Up @@ -979,7 +979,7 @@ current point. The regular expression for finding doubled words,
'the the'


Lookahead Assertions
Lookahead assertions
--------------------

Another zero-width assertion is the lookahead assertion. Lookahead assertions
Expand Down Expand Up @@ -1061,7 +1061,7 @@ end in either ``bat`` or ``exe``:
``.*[.](?!bat$|exe$)[^.]*$``


Modifying Strings
Modifying strings
=================

Up to this point, we've simply performed searches against a static string.
Expand All @@ -1083,7 +1083,7 @@ using the following pattern methods:
+------------------+-----------------------------------------------+


Splitting Strings
Splitting strings
-----------------

The :meth:`~re.Pattern.split` method of a pattern splits a string apart
Expand Down Expand Up @@ -1137,7 +1137,7 @@ argument, but is otherwise the same. ::
['Words', 'words, words.']


Search and Replace
Search and replace
------------------

Another common task is to find all the matches for a pattern, and replace them
Expand Down Expand Up @@ -1236,15 +1236,15 @@ pattern object as the first parameter, or use embedded modifiers in the
pattern string, e.g. ``sub("(?i)b+", "x", "bbbb BBBB")`` returns ``'x x'``.


Common Problems
Common problems
===============

Regular expressions are a powerful tool for some applications, but in some ways
their behaviour isn't intuitive and at times they don't behave the way you may
expect them to. This section will point out some of the most common pitfalls.


Use String Methods
Use string methods
------------------

Sometimes using the :mod:`re` module is a mistake. If you're matching a fixed
Expand Down Expand Up @@ -1310,7 +1310,7 @@ string and then backtracking to find a match for the rest of the RE. Use
:func:`re.search` instead.


Greedy versus Non-Greedy
Greedy versus non-greedy
------------------------

When repeating a regular expression, as in ``a*``, the resulting action is to
Expand Down Expand Up @@ -1388,9 +1388,9 @@ Feedback
========

Regular expressions are a complicated topic. Did this document help you
understand them? Were there parts that were unclear, or Problems you
understand them? Were there parts that were unclear, or problems you
encountered that weren't covered here? If so, please send suggestions for
improvements to the author.
improvements to the :ref:`issue tracker <using-the-tracker>`.

The most complete book on regular expressions is almost certainly Jeffrey
Friedl's Mastering Regular Expressions, published by O'Reilly. Unfortunately,
Expand Down
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