[R] Regular expressions: bug or misunderstanding?

[(Ted Harding)]

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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