R help - Jul 2008 - Regular expressions: bug or misunderstanding?

I'm trying to write a gsub() call that takes a string and escapes all 
the unescaped quote marks in it.  So the string

\"

would be left unchanged, but

\\"

would be changed to

\\\"

because the double backslash doesn't act as an escape for the quote, the 
first just escapes the second.  I have the usual problems of writing 
regular expressions involving backslashes which make everything I write 
completely unreadable, so I'm going to change the problem for this 
post:  I will define E to be the escape character, and q to be the 
quote; the gsub() call would leave

Eq

unchanged, but would change

EEq

to EEEq, etc.

The expression I have come up with after this change is

gsub( "((^|[^E])(EE)*)q", "\\1Eq", x)

i.e. "(start of line, or non-escape, followed by an even number of 
escapes), all of which we call expression 1, followed by a quote, is 
replaced by expression 1 followed by an escape and a quote".

This works sometimes, but not always:

 > gsub( "((^|[^E])(EE)*)q", "\\1Eq", "Eq")
[1] "Eq"
 > gsub( "((^|[^E])(EE)*)q", "\\1Eq", "EEq")
[1] "EEEq"
 > gsub( "((^|[^E])(EE)*)q", "\\1Eq", "qaq")
[1] "EqaEq"
 > gsub( "((^|[^E])(EE)*)q", "\\1Eq", "qq")
[1] "qEq"

Notice that in the final example, the first quote doesn't get escaped.  
Why not????

Duncan Murdoch

Try adding perl = TRUE

On Sun, Jul 6, 2008 at 5:17 PM, Duncan Murdoch <murdoch at stats.uwo.ca>
wrote:
> I'm trying to write a gsub() call that takes a string and escapes all
the
> unescaped quote marks in it.  So the string
>
> \"
>
> would be left unchanged, but
>
> \\"
>
> would be changed to
>
> \\\"
>
> because the double backslash doesn't act as an escape for the quote,
the
> first just escapes the second.  I have the usual problems of writing
regular
> expressions involving backslashes which make everything I write completely
> unreadable, so I'm going to change the problem for this post:  I will
define
> E to be the escape character, and q to be the quote; the gsub() call would
> leave
>
> Eq
>
> unchanged, but would change
>
> EEq
>
> to EEEq, etc.
>
> The expression I have come up with after this change is
>
> gsub( "((^|[^E])(EE)*)q", "\\1Eq", x)
>
> i.e. "(start of line, or non-escape, followed by an even number of
escapes),
> all of which we call expression 1, followed by a quote, is replaced by
> expression 1 followed by an escape and a quote".
>
> This works sometimes, but not always:
>
>> gsub( "((^|[^E])(EE)*)q", "\\1Eq", "Eq")
> [1] "Eq"
>> gsub( "((^|[^E])(EE)*)q", "\\1Eq", "EEq")
> [1] "EEEq"
>> gsub( "((^|[^E])(EE)*)q", "\\1Eq", "qaq")
> [1] "EqaEq"
>> gsub( "((^|[^E])(EE)*)q", "\\1Eq", "qq")
> [1] "qEq"
>
> Notice that in the final example, the first quote doesn't get escaped. 
Why
> not????
>
> Duncan Murdoch
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

On 06-Jul-08 21:17:04, Duncan Murdoch wrote:
> I'm trying to write a gsub() call that takes a string and escapes all 
> the unescaped quote marks in it.  So the string
> 
> \"
> 
> would be left unchanged, but
> 
> \\"
> 
> would be changed to
> 
> \\\"
> 
> because the double backslash doesn't act as an escape for the quote,
> the first just escapes the second.  I have the usual problems of
> writing regular expressions involving backslashes which make
> everything I write completely unreadable, so I'm going to change
> the problem for this post:  I will define E to be the escape
> character, and q to be the quote; the gsub() call would leave
> 
> Eq
> 
> unchanged, but would change
> 
> EEq
> 
> to EEEq, etc.
> 
> The expression I have come up with after this change is
> 
> gsub( "((^|[^E])(EE)*)q", "\\1Eq", x)
> 
> i.e. "(start of line, or non-escape, followed by an even number of 
> escapes), all of which we call expression 1, followed by a quote,
> is replaced by expression 1 followed by an escape and a quote".
> 
> This works sometimes, but not always:
> 
>  > gsub( "((^|[^E])(EE)*)q", "\\1Eq",
"Eq")
> [1] "Eq"
>  > gsub( "((^|[^E])(EE)*)q", "\\1Eq",
"EEq")
> [1] "EEEq"
>  > gsub( "((^|[^E])(EE)*)q", "\\1Eq",
"qaq")
> [1] "EqaEq"
>  > gsub( "((^|[^E])(EE)*)q", "\\1Eq",
"qq")
> [1] "qEq"
> 
> Notice that in the final example, the first quote doesn't get escaped. 
> Why not????
I think (without having done the "experimental diagnostics")
that it's because in "qq" the first q mtaches (^|[^E]) because
it matches [^E] (i.e. is a "non-escape"); since it is followed
by q, it is the second q which gets the escape. Possibly you
need to include "^q" as an additional alternative match at the
start of the line.

Ted.

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Date: 06-Jul-08                                       Time: 22:37:10
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