re.escape should not escape underscore

[rsc]

BPO	2650
Nosy	@birkenfeld, @amauryfa, @abalkin, @pitrou, @benjaminp, @ezio-melotti
Files	re.patch: patch to python svn trunk (2.6a2+) re.patch re.patch: patch using enumerate and frozenset re_patch.diff: Compilation of previous patches, testing for more characters included on the ASCII table test_re.diff test_re.diff issue2650.diff: Patch to add '_' to the non-escaped chars.

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ezio-melotti'
closed_at = <Date 2011-04-10.10:00:23.726>
created_at = <Date 2008-04-17.14:14:08.568>
labels = ['expert-regex', 'easy', 'type-bug']
title = 're.escape should not escape underscore'
updated_at = <Date 2011-04-10.10:00:23.726>
user = 'https://bugs.python.org/rsc'

bugs.python.org fields:

activity = <Date 2011-04-10.10:00:23.726>
actor = 'ezio.melotti'
assignee = 'ezio.melotti'
closed = True
closed_date = <Date 2011-04-10.10:00:23.726>
closer = 'ezio.melotti'
components = ['Regular Expressions']
creation = <Date 2008-04-17.14:14:08.568>
creator = 'rsc'
dependencies = []
files = ['10080', '10084', '10130', '10215', '20860', '21096', '21390']
hgrepos = []
issue_num = 2650
keywords = ['patch', 'easy']
message_count = 43.0
messages = ['65585', '65590', '65599', '65600', '65708', '65721', '65923', '66386', '66416', '66418', '66419', '66420', '66421', '66422', '66423', '68208', '68785', '68786', '68895', '68910', '92548', '122408', '125746', '125770', '125778', '126138', '126141', '126168', '126176', '126177', '126181', '126184', '126185', '130719', '130783', '130821', '130930', '132082', '132084', '132085', '132087', '132838', '133461']
nosy_count = 17.0
nosy_names = ['georg.brandl', 'amaury.forgeotdarc', 'belopolsky', 'foom', 'pitrou', 'rsc', 'timehorse', 'benjamin.peterson', 'zanella', 'donlorenzo', 'ezio.melotti', 'bjourne', 'mortenlj', 'mrabarnett', 'SilentGhost', 'swamiyeswanth', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue2650'
versions = ['Python 3.2']

[rsc]

import re
print re.escape("_")

Prints \_ but should be _.

This behavior differs from Perl and other systems: _ is an identifier
character and as such does not need to be escaped.

[benjaminp]

It seems that escape is pretty dumb. The documentations says that
re.escape escapes all non-alphanumeric characters, and it does that
faithfully. It would seem more useful to have a list of meta-characters
and just escape those. This is more true in Py3k when str can have
thousands of possible characters that could be considered alphanumeric.

[rsc]

It seems that escape is pretty dumb. The documentations says that
re.escape escapes all non-alphanumeric characters, and it does that
faithfully. It would seem more useful to have a list of meta-characters
and just escape those. This is more true in Py3k when str can have
thousands of possible characters that could be considered alphanumeric.

The usual convention is to escape everything that is
ASCII and not A-Za-z0-9_, in case other punctuation
becomes special in the future. But I agree -- escaping
just the actual special characters makes the most sense.

Russ

[benjaminp]

Would you like to work on a patch?

[benjaminp]

Thanks.

The loop in escape should really use enumerate instead of "for i in
range(len(pattern))".

Instead of using a loop, can't the test just use
"self.assertEqual(re.esacpe(same), same)?" Also, please add tests for
what re.escape should escape.

[rsc]

The loop in escape should really use enumerate
instead of "for i in range(len(pattern))".

It needs i to edit s[i].

Instead of using a loop, can't the test just
use "self.assertEqual(re.esacpe(same), same)?"

Done.

Also, please add tests for what re.escape should escape.

That's handled in the existing test over all bytes 0-255.

[donlorenzo]

> The loop in escape should really use enumerate
> instead of "for i in range(len(pattern))".

It needs i to edit s[i].

enumerate(iterable) returns a tuple for each element in iterable
containing the index and the element itself.

I attached a patch using enumerate. The patch also uses a frozenset
rather than a dict for the special characters.

[zanella]

AFAIK the lookup on dictionaries is faster than on lists.

Patch added, mainly a compilation of the previous patches with an
expanded test.

[abalkin]

Lorenz's patch uses a set, not a list for special characters. Set
lookup is as fast as dict lookup, but a set takes less memory because it
does not have to store dummy values. More importantly, use of frozenset
instead of dict makes the code clearer. On the other hand, I would
simply use a string. For a dozen entries, hash lookup does not buy you
much.

Another nit: why use "\\%c" % (c) instead of obvious "\\" + c?

Finally, you can eliminate use of index and a temporary list altogether
by using a generator expression:

''.join(("\\" + c if c in _special else '\\000' if c == "\000" else c),
for c in pattern)

[rsc]

Lorenz's patch uses a set, not a list for special characters. Set
lookup is as fast as dict lookup, but a set takes less memory because it
does not have to store dummy values. More importantly, use of frozenset
instead of dict makes the code clearer. On the other hand, I would
simply use a string. For a dozen entries, hash lookup does not buy you
much.

Another nit: why use "\\%c" % (c) instead of obvious "\\" + c?

Finally, you can eliminate use of index and a temporary list altogether
by using a generator expression:

''.join(("\\" + c if c in _special else '\\000' if c == "\000" else c),
for c in pattern)

The title of this issue (bpo-2650) is "re.escape should not escape underscore",
not "re.escape is too slow and too easy to read".

If you have an actual, measured performance problem with re.escape,
please open a new issue with numbers to back it up.
That's not what this one is about.

Thanks.
Russ

[abalkin]

On Thu, May 8, 2008 at 10:36 AM, Russ Cox <report@bugs.python.org> wrote:
..

The title of this issue (bpo-2650) is "re.escape should not escape underscore",
not "re.escape is too slow and too easy to read".

Neither does the title say "re.escape should only escape
.^$*+?{}[]\|()". I reviewed the patch rather than its conformance
with the title. (BTW, the patch does not update documentation in
Doc/library/re.rst.)

If you have an actual, measured performance problem with re.escape,
please open a new issue with numbers to back it up.
That's not what this one is about.

You don't need to get so defensive. I did not raise a performance
problem, I was simply responding to Rafael's "AFAIK the lookup on
dictionaries is faster than on lists" comment. I did not say that you
*should* rewrite your patch the way I suggested, only that you *can*
use new language features to simplify the code.

In any case, I am -0 on the patch. The current documentation says:

"""
escape(string)

Return *string* with all non-alphanumerics backslashed; this is useful if you
want to match an arbitrary literal string that may have regular expression
metacharacters in it.

"""

and the current implementation serves the intended use case well. I
did not see a compelling use case presented for the change. On the
downside, since there is no mechanism to assure that _special indeed
contains all re metacharacters, it may present a maintenance problem
if additional metacharacters are added in the future.

[rsc]

You don't need to get so defensive. I did not raise a performance
problem, I was simply responding to Rafael's "AFAIK the lookup on
dictionaries is faster than on lists" comment. I did not say that you
*should* rewrite your patch the way I suggested, only that you *can*
use new language features to simplify the code.

I was responding to the entire thread more than your mail.
I'm frustrated because the only substantial discussion has
focused on details of how to implement set lookup the fastest
in a function that likely doesn't matter for speed.

In any case, I am -0 on the patch. The current documentation says:

Now these are the kinds of comments I was hoping for.
Thank you.

Return *string* with all non-alphanumerics backslashed; this is useful if you
want to match an arbitrary literal string that may have regular expression
metacharacters in it.

Sure; the documentation is wrong too.

I did not see a compelling use case presented for the change.

The usual convention in regular expressions is that escaping
a word character means you intend a special meaning, and
underscore is a word character. Even though the current re
module does accept \_ as synonymous with _ (just as it accepts
\q as synonymous with q), it is no more correct to escape _ than
to escape q.

I think it is fine to escape all non-word characters, but someone
else suggested that it would be easier when moving to larger
character sets to escape just the special ones. I'm happy with
either version.

My argument is only that Python should behave the same in
this respect as other systems that use substantially the same
regular expressions.

since there is no mechanism to assure that _special indeed
contains all re metacharacters, it may present a maintenance problem
if additional metacharacters are added in the future.

The test suite will catch these easily, since it checks that
re.escape(c) matches c for all characters c. But again, I'm happy
with escaping all ASCII non-word characters.

Russ