[Haskell-beginners] remove XML tags using Text.Regex.Posix

[Patrick LeBoutillier]

Robert,

On Tue, Sep 29, 2009 at 3:25 PM, Robert Ziemba <rziemba at gmail.com> wrote:

> I have been working with the regular expression package (Text.Regex.Posix).
>  My hope was to find a simple way to remove a pair of XML tags from a short
> string.
>
> I have something like this "<tag>Data</tag>" and would like to extract
> 'Data'.  There is only one tag pair, no nesting, and I know exactly what the
> tag is.
>
> My first attempt was this:
>
>   "<tag>123</tag>" =~ "[^<tag>].+[^</tag>]"::String
>
> result:  "123"
>
> Upon further experimenting I realized that it only works with more than 2
> digits in 'Data'.  I occured to me that my thinking on how this regular
> expression works was not correct - but I don't understand why it works at
> all for 3 or more digits.
>
> Can anyone help me understand this result and perhaps suggest another
> strategy?  Thank you.
>

The regex you are using here can be described as such:

"Match a character not in the set '<,t,a,g,>', followed by 1 or more of
anything, followed by a character not in the set '<,/,t,a,g,>'."

Effectively, it will not match if your data has less than 3 characters and
is probably not the correct regex for this job, i.e. it would also match
"x123x". What you need is regex capturing, but I don't know if that is
available in that regex library (I'm not an expert Haskeller).

If you really need a regex to locate the tag, you could use a function like
this to extract it:

getTagData tag s =
   let match = s =~ ("<" ++ tag ++ ">.*</" ++ tag ++ ">")::String
       dropTag = drop (length tag + 2) s
       getData = take (length match - (2 * length tag + 5)) dropTag
  in if length match > 0
       then Just getData
       else Nothing

*Main> getTagData "tag" "<tag>123</tag>"
Just "123"


Patrick



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-- 
=====================
Patrick LeBoutillier
Rosemère, Québec, Canada
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