Issue #10875: Update Regular Expression HOWTO; patch by 'SilentGhost'.
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Terry Reedy
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@ -5,7 +5,6 @@
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****************************
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:Author: A.M. Kuchling <amk@amk.ca>
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:Release: 0.05
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.. TODO:
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Document lookbehind assertions
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@ -24,11 +23,6 @@
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Introduction
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============
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The :mod:`re` module was added in Python 1.5, and provides Perl-style regular
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expression patterns. Earlier versions of Python came with the :mod:`regex`
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module, which provided Emacs-style patterns. The :mod:`regex` module was
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removed completely in Python 2.5.
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Regular expressions (called REs, or regexes, or regex patterns) are essentially
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a tiny, highly specialized programming language embedded inside Python and made
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available through the :mod:`re` module. Using this little language, you specify
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@ -264,7 +258,7 @@ performing string substitutions. ::
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>>> import re
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>>> p = re.compile('ab*')
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>>> p
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<_sre.SRE_Pattern object at 80b4150>
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<_sre.SRE_Pattern object at 0x...>
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:func:`re.compile` also accepts an optional *flags* argument, used to enable
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various special features and syntax variations. We'll go over the available
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@ -362,8 +356,8 @@ information about the match: where it starts and ends, the substring it matched,
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and more.
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You can learn about this by interactively experimenting with the :mod:`re`
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module. If you have Tkinter available, you may also want to look at
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:file:`Tools/scripts/redemo.py`, a demonstration program included with the
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module. If you have :mod:`tkinter` available, you may also want to look at
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:file:`Tools/demo/redemo.py`, a demonstration program included with the
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Python distribution. It allows you to enter REs and strings, and displays
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whether the RE matches or fails. :file:`redemo.py` can be quite useful when
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trying to debug a complicated RE. Phil Schwartz's `Kodos
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@ -373,11 +367,10 @@ testing RE patterns.
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This HOWTO uses the standard Python interpreter for its examples. First, run the
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Python interpreter, import the :mod:`re` module, and compile a RE::
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Python 2.2.2 (#1, Feb 10 2003, 12:57:01)
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>>> import re
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>>> p = re.compile('[a-z]+')
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>>> p
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<_sre.SRE_Pattern object at 80c3c28>
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<_sre.SRE_Pattern object at 0x...>
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Now, you can try matching various strings against the RE ``[a-z]+``. An empty
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string shouldn't match at all, since ``+`` means 'one or more repetitions'.
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@ -395,7 +388,7 @@ result in a variable for later use. ::
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>>> m = p.match('tempo')
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>>> m
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<_sre.SRE_Match object at 80c4f68>
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<_sre.SRE_Match object at 0x...>
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Now you can query the :class:`MatchObject` for information about the matching
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string. :class:`MatchObject` instances also have several methods and
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@ -434,7 +427,7 @@ case. ::
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>>> print(p.match('::: message'))
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None
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>>> m = p.search('::: message') ; print(m)
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<re.MatchObject instance at 80c9650>
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<_sre.SRE_Match object at 0x...>
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>>> m.group()
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'message'
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>>> m.span()
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@ -459,11 +452,11 @@ Two pattern methods return all of the matches for a pattern.
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:meth:`findall` has to create the entire list before it can be returned as the
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result. The :meth:`finditer` method returns a sequence of :class:`MatchObject`
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instances as an :term:`iterator`. [#]_ ::
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instances as an :term:`iterator`::
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>>> iterator = p.finditer('12 drummers drumming, 11 ... 10 ...')
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>>> iterator
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<callable-iterator object at 0x401833ac>
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<callable_iterator object at 0x...>
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>>> for match in iterator:
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... print(match.span())
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...
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@ -485,7 +478,7 @@ the RE string added as the first argument, and still return either ``None`` or a
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>>> print(re.match(r'From\s+', 'Fromage amk'))
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None
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>>> re.match(r'From\s+', 'From amk Thu May 14 19:12:10 1998')
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<re.MatchObject instance at 80c5978>
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<_sre.SRE_Match object at 0x...>
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Under the hood, these functions simply create a pattern object for you
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and call the appropriate method on it. They also store the compiled object in a
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@ -687,7 +680,7 @@ given location, they can obviously be matched an infinite number of times.
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line, the RE to use is ``^From``. ::
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>>> print(re.search('^From', 'From Here to Eternity'))
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<re.MatchObject instance at 80c1520>
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<_sre.SRE_Match object at 0x...>
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>>> print(re.search('^From', 'Reciting From Memory'))
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None
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@ -699,11 +692,11 @@ given location, they can obviously be matched an infinite number of times.
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or any location followed by a newline character. ::
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>>> print(re.search('}$', '{block}'))
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<re.MatchObject instance at 80adfa8>
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<_sre.SRE_Match object at 0x...>
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>>> print(re.search('}$', '{block} '))
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None
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>>> print(re.search('}$', '{block}\n'))
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<re.MatchObject instance at 80adfa8>
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<_sre.SRE_Match object at 0x...>
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To match a literal ``'$'``, use ``\$`` or enclose it inside a character class,
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as in ``[$]``.
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@ -728,7 +721,7 @@ given location, they can obviously be matched an infinite number of times.
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>>> p = re.compile(r'\bclass\b')
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>>> print(p.search('no class at all'))
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<re.MatchObject instance at 80c8f28>
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<_sre.SRE_Match object at 0x...>
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>>> print(p.search('the declassified algorithm'))
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None
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>>> print(p.search('one subclass is'))
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@ -746,7 +739,7 @@ given location, they can obviously be matched an infinite number of times.
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>>> print(p.search('no class at all'))
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None
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>>> print(p.search('\b' + 'class' + '\b') )
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<re.MatchObject instance at 80c3ee0>
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<_sre.SRE_Match object at 0x...>
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Second, inside a character class, where there's no use for this assertion,
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``\b`` represents the backspace character, for compatibility with Python's
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@ -1316,8 +1309,8 @@ a regular expression that handles all of the possible cases, the patterns will
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be *very* complicated. Use an HTML or XML parser module for such tasks.)
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Not Using re.VERBOSE
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--------------------
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Using re.VERBOSE
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----------------
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By now you've probably noticed that regular expressions are a very compact
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notation, but they're not terribly readable. REs of moderate complexity can
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@ -1366,8 +1359,3 @@ reference for programming in Python. (The first edition covered Python's
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now-removed :mod:`regex` module, which won't help you much.) Consider checking
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it out from your library.
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.. rubric:: Footnotes
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.. [#] Introduced in Python 2.2.2.
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