[R] Matching backslash in a table's column using R language

Peter Bishop
Tue, 25 Aug 2020 19:16:35 -0000

The feed is coming from a SQL table and this is using the embedded support for R
which comes with SQL 2016. The source is therefore a SELECT statement.


As an aside, I found a workaround by changing the pattern from:


"[\x22\x27\x2c\x3f\x5c\x60]"


to:


"[\x22\x27\x2c\x3f\x5c\x5c\x60]"


This seems to be escaping the backslash in the R script rather than in the data
- which confuses me.

________________________________
From: Bert Gunter <bgunter.4567 at gmail.com>
Sent: Wednesday, 26 August 2020 4:26 AM
To: Peter Bishop <bishop_peterj at hotmail.com>
Cc: r-help at r-project.org <r-help at r-project.org>
Subject: Re: [R] Matching backslash in a table's column using R language

1. I am far from an expert on such matters
2. It is unclear to me what your input is -- I assume a file.

The problem, as you indicate, is that R's parser sees "\B" as an
incorrect escape character, so, for example:
> cat("\B")
Error: '\B' is an unrecognized escape in character string starting
""\B"

In any case, I think you should look at ?scan. Here is an example where I scan
from the keyboard first and then remove the "\". You may have to scan
from a file to do this.

> z <-scan(file = "", what = "character")
1: A\BCDEFG
2:     #CR terminates input
Read 1 item

> cat(z)
A\BCDEFG

> nchar(z)
[1] 8  ## scan read in the "\" as a single character from the console.

> sub("\\\\","",z)  ## Yes, 4 backslashes
[1] "ABCDEFG"

There may be better ways to do this, but as I said, I'm no expert.

BTW, in posting here, please post in *plain text,* as the server can mangle
html.

Cheers,
Bert


Bert Gunter

"The trouble with having an open mind is that people keep coming along and
sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Tue, Aug 25, 2020 at 9:02 AM Peter Bishop <bishop_peterj at
hotmail.com<mailto:bishop_peterj at hotmail.com>> wrote:
In SQL, I'm using R as a way to filter data based on:
    - 20 characters in the range <space> to <tilde>
    - excluding <quote>, <apostrophe>, <comma>, <question
mark>, <backslash>, <backtick>

Given a SQL column containing the data:

    code
    ----
    A\BCDEFG

and the T-SQL script:

    EXEC [sys].[sp_execute_external_script]
            @language=N'R',
            @script=N'
    pattern1 = "^[\x20-\x7e]{1,20}$"
    pattern2 = "[\x22\x27\x2c\x3f\x5c\x60]"

    outData <- subset(inData, grepl(pattern1, code, perl=TRUE) &
!grepl(pattern2, code, perl=TRUE))',
            @input_data_1 = N'SELECT [code] FROM [dbo].[products]',
            @input_data_1_name = N'inData',
            @output_data_1_name = N'outData'
    WITH
            RESULT SETS (AS OBJECT [dbo].[products]);
    GO

why does the row detailed above get returned? I know that backslash is a special
character but not in the SQL table. Consequently, the T-SQL code:

    SELECT ASCII(SUBSTRING([value], 2, 1)) FROM [table]

returns 92 (the ASCII code for <backslash>) which shows that this is being
recognised as a backslash character and not as an escape indicator for the
following "B".

Can anyone advise how I can filter out the <backslash> in the way that the
other identified characters are being successfully filtered? As the data is
being retrieved from a table, I can?t ask the data provider to use ?\\? instead
of ?\? as that will be invalid for other uses.

Thanks.

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