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devel / comp.lang.awk / Re: gsub() & Escaped characters in strings

SubjectAuthor
* Re: gsub() & Escaped characters in stringsEd Morton
`* Re: gsub() & Escaped characters in stringsOğuz
 `* Re: gsub() & Escaped characters in stringsOğuz
  `- Re: gsub() & Escaped characters in stringsLuuk

1
Re: gsub() & Escaped characters in strings

<s6ejp4$fn9$1@dont-email.me>

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From: mortonspam@gmail.com (Ed Morton)
Newsgroups: comp.lang.awk
Subject: Re: gsub() & Escaped characters in strings
Date: Thu, 29 Apr 2021 10:36:04 -0500
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 by: Ed Morton - Thu, 29 Apr 2021 15:36 UTC

On 4/28/2021 8:37 PM, J Naman wrote:
> # I do not understand why gsub() seems to quadruple scan escaped characters inside
> # strings when the number of escaped characters is > 6. See below.
> BEGIN{ # quick test of regexpr
> # to match path = ...\foo\...
> # (path ~ "\\\\foo\\\\") is required
> # looking for "\\\\foo\\\\" string constant <==> regexp /\\foo\\/
> # in every case below, gsub() returns 2 = number of substitutions (as expected)
> x=";foo;"; n=gsub(/;/,"\\",x); printf("# returns %s\n",x)
> x=";foo;"; n=gsub(/;/,"\\\\",x); printf("# returns %s\n",x)
> x=";foo;"; n=gsub(/;/,"\\\\\\",x); printf("# returns %s\n",x)
> x=";foo;"; n=gsub(/;/,"\\\\\\\\",x); printf("# returns %s\n",x)
> x=";foo;"; n=gsub(/;/,"\\\\\\\\\\",x); printf("# returns %s\n",x)
> x=";foo;"; n=gsub(/;/,"\\\\\\\\\\\\",x); printf("# returns %s\n",x)
> }
> # 2 \s returns 1 escaped: \foo\ as expected
> # 4 \s returns 2 escaped: \\foo\\ as expected
> # 6 \s returns 3 escaped: \\\foo\\\ as expected
> # 8 \s returns 2 escaped: \\foo\\ NOT 4 \s
> # 10 \s returns 3 escaped: \\\foo\\\ NOT 6 \s
> # 12 \s returns 4 escaped: \\\\foo\\\\ ! finally get 4 \s!
> # Can anyone explain why 8+ \s are different?
> # Why gsub(/;/,"\\\\",x) == gsub(/;/,"\\\\\\\\",x)
> Thanks, john
>

I'm _guessing_ it's because the string gets interpreted twice, once when
the awk interpreter reads it and then again when it uses it, so for the
2 passes of interpretation, depending on how the "use" phase interprets
pairs of backslashes, we could get:

\\ -> read -> \ -> use -> \
\\\\ -> read -> \\ -> use -> \ or \\
\\\\\\ -> read -> \\\ -> use -> \\\
\\\\\\\\ -> read -> \\\\ -> use -> \\ or \\\\

Now WHY any given awk when using the string would interpret 4
backslashes as 2 but not 2 backslashes as 1, I can't guess.

Regards,

Ed.

Re: gsub() & Escaped characters in strings

<28b93171-68ae-4aab-a00b-2be4c8d7cfden@googlegroups.com>

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Subject: Re: gsub() & Escaped characters in strings
From: oguzismailuysal@gmail.com (Oğuz)
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 by: Oğuz - Thu, 29 Apr 2021 17:33 UTC

On Thursday, April 29, 2021 at 6:36:06 PM UTC+3, Ed Morton wrote:
> On 4/28/2021 8:37 PM, J Naman wrote:
> > # I do not understand why gsub() seems to quadruple scan escaped characters inside
> > # strings when the number of escaped characters is > 6. See below.
> > BEGIN{ # quick test of regexpr
> > # to match path = ...\foo\...
> > # (path ~ "\\\\foo\\\\") is required
> > # looking for "\\\\foo\\\\" string constant <==> regexp /\\foo\\/
> > # in every case below, gsub() returns 2 = number of substitutions (as expected)
> > x=";foo;"; n=gsub(/;/,"\\",x); printf("# returns %s\n",x)
> > x=";foo;"; n=gsub(/;/,"\\\\",x); printf("# returns %s\n",x)
> > x=";foo;"; n=gsub(/;/,"\\\\\\",x); printf("# returns %s\n",x)
> > x=";foo;"; n=gsub(/;/,"\\\\\\\\",x); printf("# returns %s\n",x)
> > x=";foo;"; n=gsub(/;/,"\\\\\\\\\\",x); printf("# returns %s\n",x)
> > x=";foo;"; n=gsub(/;/,"\\\\\\\\\\\\",x); printf("# returns %s\n",x)
> > }
> > # 2 \s returns 1 escaped: \foo\ as expected
> > # 4 \s returns 2 escaped: \\foo\\ as expected
> > # 6 \s returns 3 escaped: \\\foo\\\ as expected
> > # 8 \s returns 2 escaped: \\foo\\ NOT 4 \s
> > # 10 \s returns 3 escaped: \\\foo\\\ NOT 6 \s
> > # 12 \s returns 4 escaped: \\\\foo\\\\ ! finally get 4 \s!
> > # Can anyone explain why 8+ \s are different?
> > # Why gsub(/;/,"\\\\",x) == gsub(/;/,"\\\\\\\\",x)
> > Thanks, john
> >
> I'm _guessing_ it's because the string gets interpreted twice, once when
> the awk interpreter reads it and then again when it uses it, so for the
> 2 passes of interpretation, depending on how the "use" phase interprets
> pairs of backslashes, we could get:
>
> \\ -> read -> \ -> use -> \
> \\\\ -> read -> \\ -> use -> \ or \\
> \\\\\\ -> read -> \\\ -> use -> \\\
> \\\\\\\\ -> read -> \\\\ -> use -> \\ or \\\\

According to POSIX, when used as the second argument to sub or gsub, "\\" and "\\\\" are the same (one backslash) unless the former is followed by an ampersand. The same goes for "\\\\\\" and "\\\\\\\\" (two backslashes). Like, given the following program,
BEGIN { x = "y"; sub(/y/, "\\y\\\\y\\\\\\y\\\\\\\\y", x); print x }
a POSIX-conformant awk should output:
\y\y\\y\\y

Now, I have busybox awk, gawk, mawk, nawk, and NetBSD awk installed on my computer, and none of them gives that output.

>
> Now WHY any given awk when using the string would interpret 4
> backslashes as 2 but not 2 backslashes as 1, I can't guess.
>
> Regards,
>
> Ed.

Re: gsub() & Escaped characters in strings

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Subject: Re: gsub() & Escaped characters in strings
From: oguzismailuysal@gmail.com (Oğuz)
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 by: Oğuz - Thu, 29 Apr 2021 17:35 UTC

On Thursday, April 29, 2021 at 8:33:21 PM UTC+3, Oğuz wrote:
> Now, I have busybox awk, gawk, mawk, nawk, and NetBSD awk installed on my computer, and none of them gives that output.
No, sorry, actually mawk does.
$ mawk 'BEGIN { x = "y"; sub(/y/, "\\y\\\\y\\\\\\y\\\\\\\\y", x); print x }'
\y\y\\y\\y

Re: gsub() & Escaped characters in strings

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Subject: Re: gsub() & Escaped characters in strings
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From: luuk@invalid.lan (Luuk)
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 by: Luuk - Thu, 29 Apr 2021 19:19 UTC

On 29-4-2021 19:35, Oğuz wrote:
> On Thursday, April 29, 2021 at 8:33:21 PM UTC+3, Oğuz wrote:
>> Now, I have busybox awk, gawk, mawk, nawk, and NetBSD awk installed on my computer, and none of them gives that output.
> No, sorry, actually mawk does.
> $ mawk 'BEGIN { x = "y"; sub(/y/, "\\y\\\\y\\\\\\y\\\\\\\\y", x); print x }'
> \y\y\\y\\y
>

D:\TEMP>gawk "BEGIN { x = \"y\"; sub(/y/, \"\\y\\\\y\\\\\\y\\\\\\\\y\",
x); print x }"
\y\\y\\\y\\y

D:\TEMP>gawk -P "BEGIN { x = \"y\"; sub(/y/,
\"\\y\\\\y\\\\\\y\\\\\\\\y\", x); print x }"
\y\y\\y\\y

D:\TEMP>gawk --version
GNU Awk 5.1.0, API: 3.0 (GNU MPFR 3.1.5, GNU MP 6.1.2)
Copyright (C) 1989, 1991-2020 Free Software Foundation.

This pro.......

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