On 2024-10-07 14:35, Michael F. Stemper via Python-list wrote:

I'm trying to discard lines that include the string "\sout{" (which is TeX, for
those who are curious. I have tried:
if not re.search("\sout{", line):
if not re.search("\sout\{", line):
if not re.search("\\sout{", line):
if not re.search("\\sout\{", line):

But the lines with that string keep coming through. What is the right syntax to
properly escape the backslash and the left curly bracket?

String literals use backslash is an escape character, so it needs to be escaped, or you need to use a "raw" string.

However, regex also uses backslash as an escape character.

That means that a literal backslash in a regex that's in a plain string
literal needs to be doubly-escaped, once for the string literal and
again for the regex.

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On 2024-10-08 19:30, Karsten Hilbert via Python-list wrote:

Am Mon, Oct 07, 2024 at 08:35:32AM -0500 schrieb Michael F. Stemper via Python-list:

I'm trying to discard lines that include the string "\sout{" (which is TeX, for
those who are curious. I have tried:
if not re.search("\sout{", line):
if not re.search("\sout\{", line):
if not re.search("\\sout{", line):
if not re.search("\\sout\{", line):

unwanted_tex = '\sout{'
if unwanted_tex not in line: do_something_with_libreoffice()

That should be:

unwanted_tex = r'\sout{'

or:

unwanted_tex = '\\sout{'

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Am Mon, Oct 07, 2024 at 08:35:32AM -0500 schrieb Michael F. Stemper via Python-list:

I'm trying to discard lines that include the string "\sout{" (which is TeX, for
those who are curious. I have tried:
if not re.search("\sout{", line):
if not re.search("\sout\{", line):
if not re.search("\\sout{", line):
if not re.search("\\sout\{", line):

unwanted_tex = '\sout{'
if unwanted_tex not in line: do_something_with_libreoffice()

Karsten
--
GPG 40BE 5B0E C98E 1713 AFA6 5BC0 3BEA AC80 7D4F C89B

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ram@zedat.fu-berlin.de (Stefan Ram) wrote or quoted:

"\\\\chardef \\\\\\\\ = '\\\\\\\\".

However, one can rewrite this as follows:

"`chardef `` = '``".replace( "`", "\\"*4 )

. One can also use "repr" to find how to represent something:

main.py

text = input( "What do you want me to represent as a literal? " )
print( repr( text ))

transcript

What do you want me to represent as a literal? \\sout\{
'\\\\sout\\{'

. We can use "escape" and "repr" to find how to represent
a regular expression for a literal text:

main.py

import re

text = input( "Want the literal of an re for what text? " )
print( repr( re.escape( text )))

transcript

Want the literal of an re for what text? \sout{
'\\\\sout\\{'

.

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MRAB <python@mrabarnett.plus.com> wrote or quoted:

However, regex also uses backslash as an escape character.

TeX also uses the backslash as an escape character:

\chardef \\ = '\\

, the regular expression to search exactly this:

\\chardef \\\\ = '\\\\

, and the Python string literal for that regular expression:

"\\\\chardef \\\\\\\\ = '\\\\\\\\".

. Must be a reason Markdown started to use the backtick!

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Karsten Hilbert <Karsten.Hilbert@gmx.net> writes:

Python 3.11.2 (main, Aug 26 2024, 07:20:54) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> tex = '\sout{'
>>> tex
'\\sout{'
>>>

Am I missing something ?

You're missing the warning it generates:

> python -E -Wonce
Python 3.11.2 (main, Aug 26 2024, 07:20:54) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> tex = '\sout{'
<stdin>:1: DeprecationWarning: invalid escape sequence '\s'
>>>

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Am Tue, Oct 08, 2024 at 08:07:04PM +0100 schrieb MRAB via Python-list:

unwanted_tex = '\sout{'
if unwanted_tex not in line: do_something_with_libreoffice()

That should be:

unwanted_tex = r'\sout{'

Hm.

Python 3.11.2 (main, Aug 26 2024, 07:20:54) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

tex = '\sout{'
tex

'\\sout{'

Am I missing something ?

Karsten
--
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On 2024-10-08 21:59, Alan Bawden via Python-list wrote:

Karsten Hilbert <Karsten.Hilbert@gmx.net> writes:

Python 3.11.2 (main, Aug 26 2024, 07:20:54) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> tex = '\sout{'
>>> tex
'\\sout{'
>>>

Am I missing something ?

You're missing the warning it generates:

> python -E -Wonce
Python 3.11.2 (main, Aug 26 2024, 07:20:54) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> tex = '\sout{'
<stdin>:1: DeprecationWarning: invalid escape sequence '\s'
>>>

You got lucky that \s in invalid. If it had been \t you would've got a
tab character.

Historically, Python treated invalid escape sequences as literals, but
it's deprecated now and will become an outright error in the future
(probably) because it often hides a mistake, such as the aforementioned
\t being treated as a tab character when the user expected it to be a
literal backslash followed by letter t. (This can occur within Windows
file paths written in plain string literals.)

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Am Tue, Oct 08, 2024 at 04:59:48PM -0400 schrieb Alan Bawden via Python-list:

Karsten Hilbert <Karsten.Hilbert@gmx.net> writes:

Python 3.11.2 (main, Aug 26 2024, 07:20:54) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> tex = '\sout{'
>>> tex
'\\sout{'
>>>

Am I missing something ?

You're missing the warning it generates:

<stdin>:1: DeprecationWarning: invalid escape sequence '\s'

I knew it'd be good to ask :-D

Karsten
--
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On Mon, 7 Oct 2024 08:35:32 -0500, Michael F. Stemper wrote:

I'm trying to discard lines that include the string "\sout{" (which is
TeX, for those who are curious. I have tried:
if not re.search("\sout{", line): if not re.search("\sout\{", line):
if not re.search("\\sout{", line): if not re.search("\\sout\{",
line):

But the lines with that string keep coming through. What is the right
syntax to properly escape the backslash and the left curly bracket?

$ python
Python 3.12.6 (main, Sep 8 2024, 13:18:56) [GCC 14.2.1 20240805] on linux
Type "help", "copyright", "credits" or "license" for more information.

import re
s = r"testing \sout{WHADDEVVA}"
re.search(r"\\sout{", s)

<re.Match object; span=(8, 14), match='\\sout{'>

You want a literal backslash, hence, you need to escape everything.

It is not enough to escape the "\s" as "\\s", because that only takes care
of Python's demands for escaping "\". You also need to escape the "\" for
the RegEx as well, or it will read it like it means "\s", which is the
RegEx for a space character and therefore your search doesn't match,
because it reads it like you want to search for " out{".

Therefore, you need to escape it either as per my example, or by using
four "\" and no "r" in front of the first quote, which also works:

re.search("\\\\sout{", s)

<re.Match object; span=(8, 14), match='\\sout{'>

You don't need to escape the curly braces. We call them "seagull wings"
where I live.

--
Gilmeh

Sometimes I simply feel that the whole world is a cigarette and I'm the
only ashtray.

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On 2024-10-11 22:13, AVI GROSS via Python-list wrote:

Is there some utility function out there that can be called to show what the regular expression you typed in will look like by the time it is ready to be used?

Obviously, life is not that simple as it can go through multiple layers with each dealing with a layer of backslashes.

But for simple cases, ...

Yes. It's called 'print'. :-)

-----Original Message-----
From: Python-list <python-list-bounces+avi.e.gross=gmail.com@python.org> On Behalf Of Gilmeh Serda via Python-list
Sent: Friday, October 11, 2024 10:44 AM
To: python-list@python.org
Subject: Re: Correct syntax for pathological re.search()

On Mon, 7 Oct 2024 08:35:32 -0500, Michael F. Stemper wrote:

I'm trying to discard lines that include the string "\sout{" (which is
TeX, for those who are curious. I have tried:
if not re.search("\sout{", line): if not re.search("\sout\{", line):
if not re.search("\\sout{", line): if not re.search("\\sout\{",
line):

But the lines with that string keep coming through. What is the right
syntax to properly escape the backslash and the left curly bracket?

$ python
Python 3.12.6 (main, Sep 8 2024, 13:18:56) [GCC 14.2.1 20240805] on linux Type "help", "copyright", "credits" or "license" for more information.

import re
s = r"testing \sout{WHADDEVVA}"
re.search(r"\\sout{", s)

<re.Match object; span=(8, 14), match='\\sout{'>

You want a literal backslash, hence, you need to escape everything.

It is not enough to escape the "\s" as "\\s", because that only takes care
of Python's demands for escaping "\". You also need to escape the "\" for
the RegEx as well, or it will read it like it means "\s", which is the
RegEx for a space character and therefore your search doesn't match,
because it reads it like you want to search for " out{".

Therefore, you need to escape it either as per my example, or by using
four "\" and no "r" in front of the first quote, which also works:

re.search("\\\\sout{", s)

<re.Match object; span=(8, 14), match='\\sout{'>

You don't need to escape the curly braces. We call them "seagull wings"
where I live.

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On 2024-10-12 08:51:57 -0400, Thomas Passin via Python-list wrote:

On 10/12/2024 6:59 AM, Peter J. Holzer via Python-list wrote:

On 2024-10-11 17:13:07 -0400, AVI GROSS via Python-list wrote:

Is there some utility function out there that can be called to show what the
regular expression you typed in will look like by the time it is ready to be
used?

I assume that by "ready to be used" you mean the compiled form?

No, there doesn't seem to be a way to dump that. You can

p = re.compile("\\\\sout{")
print(p.pattern)

but that just prints the input string, which you could do without
compiling it first.

It prints the escaped version,

Did you mean the *un*escaped version? Well, yeah, that's what print
does.

so you can see if you escaped the string as you intended. In this
case, the print will display '\\sout{'.

print("\\\\sout{")
will do the same.

It seems to me that for any string s which is a valid regular expression
(i.e. re.compile doesn't throw an exception)

assert re.compile(s).pattern == s

holds.

So it doesn't give you anything you didn't already know.

As a trivial example, the regular expressions r"\\sout{" and r"\\sout\{"
are equivalent (the \ before the { is redundant). Yet
re.compile(s).pattern preserves the difference between the two strings.

hp

--
_ | Peter J. Holzer | Story must make more sense than reality.
|_|_) | |
| | | hjp@hjp.at | -- Charles Stross, "Creative writing
__/ | http://www.hjp.at/ | challenge!"

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Peter J. Holzer ha scritto:

As a trivial example, the regular expressions r"\\sout{" and r"\\sout\{"
are equivalent (the \ before the { is redundant). Yet
re.compile(s).pattern preserves the difference between the two strings.

Hi,
Allow me to be fussy: r"\\sout{" and r"\\sout\{" are similar but not equivalent. If you omit the backslash, the parser will have to determine
if the graph is part of regular expression {n, m} and will take more
time. In some online regexs have these results:

r"\\sout{" : 1 match ( 7 steps, 620 μs )

r"\\sout\{" : 1 match ( 7 steps, 360 μs )

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