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. -------------------------------------------------------------------- E-Mail: (Ted Harding) <[EMAIL PROTECTED]> Fax-to-email: +44 (0)870 094 0861 Date: 06-Jul-08 Time: 22:37:10 ------------------------------ XFMail ------------------------------ ______________________________________________ R-help@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.
